[clang][OpenMP] Assertion `!FD->getType().isNull() && "Expected a valid FunctionDecl"' failed.

Reproducer:
https://godbolt.org/z/WcavTqrbq
```cpp
template void foo(Args... args) {
auto c = []() -> decltype(({
#pragma omp target parallel for
for (int i = 0; i < 2; i++)
;
})) {};
}
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/AST/Decl.cpp:6108: bool clang::IsArmStreamingFunction(const clang::FunctionDecl*, bool): Assertion `!FD->getType().isNull() && "Expected a valid FunctionDecl"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and dumped files.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -fopenmp
1. :4:5: current parser token 'for'
2. :1:52: parsing function body 'foo'
3. :1:52: in compound statement ('{}')
4. :2:12: lambda expression parsing
5. :2:30: in compound statement ('{}')
#0 0x00000000043f3668 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f3668)
#1 0x00000000043f0524 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f0524)
#2 0x00000000043f0b4c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f0b4c)
#3 0x000000000432fb88 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x000075ca92645330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x000075ca9269eb2c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb2c)
#6 0x000075ca9264527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x000075ca926288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x000075ca9262881b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
#9 0x000075ca9263b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#10 0x0000000007f1ecd6 clang::IsArmStreamingFunction(clang::FunctionDecl const*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7f1ecd6)
#11 0x0000000007683dbf clang::Sema::ActOnCapturedRegionStart(clang::SourceLocation, clang::Scope*, clang::CapturedRegionKind, llvm::ArrayRef>, unsigned int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7683dbf)
#12 0x00000000074f625d processCapturedRegions(clang::Sema&, llvm::omp::Directive, clang::Scope*, clang::SourceLocation) SemaOpenMP.cpp:0:0
#13 0x00000000074f635f clang::SemaOpenMP::ActOnOpenMPRegionStart(llvm::omp::Directive, clang::Scope*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x74f635f)
#14 0x0000000006ddb516 clang::Parser::ParseOpenMPExecutableDirective(clang::Parser::ParsedStmtContext, llvm::omp::Directive, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ddb516)
#15 0x0000000006de65b5 clang::Parser::ParseOpenMPDeclarativeOrExecutableDirective(clang::Parser::ParsedStmtContext, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6de65b5)
#16 0x0000000006e070f4 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e070f4)
#17 0x0000000006e08959 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e08959)
#18 0x0000000006e10dee clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e10dee)
#19 0x0000000006e11534 void llvm::function_ref::callback_fn(long) ParseStmt.cpp:0:0
#20 0x00000000087fec81 clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x87fec81)
#21 0x0000000006e011da clang::Parser::ParseCompoundStatement(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e011da)
#22 0x0000000006d895d9 clang::Parser::ParseParenExpression(clang::ParenParseOption&, bool, clang::ParenExprKind, clang::TypoCorrectionTypeBehavior, clang::OpaquePtr&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d895d9)
#23 0x0000000006d7f7bd clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d7f7bd)
#24 0x0000000006d80967 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d80967)
#25 0x0000000006d809f9 clang::Parser::ParseAssignmentExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d809f9)
#26 0x0000000006d852e9 clang::Parser::ParseExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d852e9)
#27 0x0000000006d60485 clang::Parser::ParseDecltypeSpecifier(clang::DeclSpec&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d60485)
#28 0x0000000006d9db52 clang::Parser::ParseOptionalCXXScopeSpecifier(clang::CXXScopeSpec&, clang::OpaquePtr, bool, bool, bool*, bool, clang::IdentifierInfo const**, bool, bool, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d9db52)
#29 0x0000000006d06800 clang::Parser::TryAnnotateCXXScopeToken(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d06800)
#30 0x0000000006d47e14 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d47e14)
#31 0x0000000006d49905 clang::Parser::ParseSpecifierQualifierList(clang::DeclSpec&, clang::ImplicitTypenameContext, clang::AccessSpecifier, clang::Parser::DeclSpecContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d49905)
#32 0x0000000006d49e05 clang::Parser::ParseTypeName(clang::SourceRange*, clang::DeclaratorContext, clang::AccessSpecifier, clang::Decl**, clang::ParsedAttributes*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d49e05)
#33 0x0000000006d9b3af clang::Parser::ParseLambdaExpressionAfterIntroducer(clang::LambdaIntroducer&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d9b3af)
#34 0x0000000006d9b7bb clang::Parser::ParseLambdaExpression() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d9b7bb)
#35 0x0000000006d7ffa5 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d7ffa5)
#36 0x0000000006d80967 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d80967)
#37 0x0000000006d809f9 clang::Parser::ParseAssignmentExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d809f9)
#38 0x0000000006daa290 clang::Parser::ParseInitializer(clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6daa290)
#39 0x0000000006d385ff clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d385ff)
#40 0x0000000006d587f9 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d587f9)
#41 0x0000000006d59c3f clang::Parser::ParseSimpleDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, bool, clang::Parser::ForRangeInit*, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d59c3f)
#42 0x0000000006d5a10c clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d5a10c)
#43 0x0000000006e080ad clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e080ad)
#44 0x0000000006e08959 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e08959)
#45 0x0000000006e10dee clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e10dee)
#46 0x0000000006e1160a clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e1160a)
#47 0x0000000006d0f973 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d0f973)
#48 0x0000000006d59266 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d59266)
#49 0x0000000006e1d180 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e1d180)
#50 0x0000000006e297f3 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#51 0x0000000006e29bea clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e29bea)
#52 0x0000000006d5a2b0 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d5a2b0)
#53 0x0000000006d10df6 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d10df6)
#54 0x0000000006d11ce5 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d11ce5)
#55 0x0000000006d120d0 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d120d0)
#56 0x0000000006ceebc3 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ceebc3)
#57 0x0000000004ef1628 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ef1628)
#58 0x000000000521ccc8 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x521ccc8)
#59 0x00000000051aa16d clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51aa16d)
#60 0x00000000052f53fd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52f53fd)
#61 0x0000000000da6c9c cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda6c9c)
#62 0x0000000000d9d8aa ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#63 0x0000000000d9da2d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#64 0x0000000004f688f9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#65 0x000000000432ffc4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x432ffc4)
#66 0x0000000004f69465 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#67 0x0000000004f21e32 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f21e32)
#68 0x0000000004f26667 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f26667)
#69 0x0000000004f30bc5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f30bc5)
#70 0x0000000000da318a clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda318a)
#71 0x0000000000cbef4a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcbef4a)
#72 0x000075ca9262a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#73 0x000075ca9262a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#74 0x0000000000d9d255 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd9d255)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```

Reproduce the assertion with the linked Compiler Explorer example using clang++ -fopenmp. Start with clang/lib/AST/Decl.cpp at IsArmStreamingFunction and follow the call from clang/lib/Sema/SemaOpenMP.cpp through ActOnCapturedRegionStart. Done means the reproducer no longer aborts with the invalid FunctionDecl assertion and has regression coverage.

Written by the indexing model from the issue text.