[Clang] Assertion failure in QualType::getCommonPtr() when parsing block literal with return statement in template argument context

Generated by fuzzer.
Compiler Explorer: https://godbolt.org/z/zM3GKnTsa

`x{a(b(c<(^(d<({return 0;})>){}),e));}`

Backtrace:

```
clang++: /root/llvm-project/llvm/tools/clang/include/clang/AST/TypeBase.h:953: const clang::ExtQualsTypeCommonBase* clang::QualType::getCommonPtr() const: Assertion `!isNull() && "Cannot retrieve a NULL type pointer"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
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
1. :1:24: current parser token ';'
2. :1:10: block literal parsing
3. :1:15: in compound statement ('{}')
#0 0x000000000430abe8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x430abe8)
#1 0x0000000004308014 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4308014)
#2 0x000000000424aa08 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x00007090d9842520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x00007090d98969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x00007090d9842476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x00007090d98287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x00007090d982871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x00007090d9839e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x00000000073b7a82 clang::Sema::ActOnCapScopeReturnStmt(clang::SourceLocation, clang::Expr*, clang::Sema::NamedReturnInfo&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x73b7a82)
#10 0x00000000073b80ba clang::Sema::BuildReturnStmt(clang::SourceLocation, clang::Expr*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x73b80ba)
#11 0x00000000073b95c7 clang::Sema::ActOnReturnStmt(clang::SourceLocation, clang::Expr*, clang::Scope*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x73b95c7)
#12 0x0000000006b4daba clang::Parser::ParseReturnStatement() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b4daba)
#13 0x0000000006b51efd clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b51efd)
#14 0x0000000006b52e16 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b52e16)
#15 0x0000000006b5b75e clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b5b75e)
#16 0x0000000006b5bed4 void llvm::function_ref::callback_fn(long) ParseStmt.cpp:0:0
#17 0x0000000008539031 clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x8539031)
#18 0x0000000006b4c88a clang::Parser::ParseCompoundStatement(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b4c88a)
#19 0x0000000006ad98b1 clang::Parser::ParseParenExpression(clang::ParenParseOption&, bool, clang::ParenExprKind, clang::TypoCorrectionTypeBehavior, clang::OpaquePtr&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad98b1)
#20 0x0000000006acfcd8 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6acfcd8)
#21 0x0000000006adbffe clang::Parser::ParseConstantExpressionInExprEvalContext(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6adbffe)
#22 0x0000000006b6779f clang::Parser::ParseTemplateArgument() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b6779f)
#23 0x0000000006b703c5 clang::Parser::ParseTemplateArgumentList(llvm::SmallVector&, clang::OpaquePtr, clang::SourceLocation) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b703c5)
#24 0x0000000006b70827 clang::Parser::ParseTemplateIdAfterTemplateName(bool, clang::SourceLocation&, llvm::SmallVector&, clang::SourceLocation&, clang::OpaquePtr) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b70827)
#25 0x0000000006b709ff clang::Parser::AnnotateTemplateIdToken(clang::OpaquePtr, clang::TemplateNameKind, clang::CXXScopeSpec&, clang::SourceLocation, clang::UnqualifiedId&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b709ff)
#26 0x0000000006ae88f2 clang::Parser::ParseOptionalCXXScopeSpecifier(clang::CXXScopeSpec&, clang::OpaquePtr, bool, bool, bool*, bool, clang::IdentifierInfo const**, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ae88f2)
#27 0x0000000006a57523 clang::Parser::TryAnnotateTypeOrScopeToken(clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a57523)
#28 0x0000000006a83943 clang::Parser::isDeclarationSpecifier(clang::ImplicitTypenameContext, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a83943)
#29 0x0000000006aa37c2 clang::Parser::ParseParenDeclarator(clang::Declarator&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6aa37c2)
#30 0x0000000006acd383 clang::Parser::ParseBlockLiteralExpression() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6acd383)
#31 0x0000000006acf91a clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6acf91a)
#32 0x0000000006ad1047 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad1047)
#33 0x0000000006ad10d9 clang::Parser::ParseAssignmentExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad10d9)
#34 0x0000000006ad56b9 clang::Parser::ParseExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad56b9)
#35 0x0000000006ad9494 clang::Parser::ParseParenExpression(clang::ParenParseOption&, bool, clang::ParenExprKind, clang::TypoCorrectionTypeBehavior, clang::OpaquePtr&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad9494)
#36 0x0000000006acfcd8 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6acfcd8)
#37 0x0000000006ad1047 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad1047)
#38 0x0000000006ad2116 clang::Parser::ParseRHSOfBinaryExpression(clang::ActionResult, clang::prec::Level) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad2116)
#39 0x0000000006ad143e clang::Parser::ParseExpressionList(llvm::SmallVectorImpl&, llvm::function_ref, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad143e)
#40 0x0000000006ad619d clang::Parser::ParsePostfixExpressionSuffix(clang::ActionResult) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad619d)
#41 0x0000000006acedea clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6acedea)
#42 0x0000000006ad1047 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad1047)
#43 0x0000000006ad10d9 clang::Parser::ParseAssignmentExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad10d9)
#44 0x0000000006ad143e clang::Parser::ParseExpressionList(llvm::SmallVectorImpl&, llvm::function_ref, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad143e)
#45 0x0000000006ad619d clang::Parser::ParsePostfixExpressionSuffix(clang::ActionResult) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad619d)
#46 0x0000000006acedea clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6acedea)
#47 0x0000000006ad1047 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad1047)
#48 0x0000000006ad10d9 clang::Parser::ParseAssignmentExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ad10d9)
#49 0x0000000006af9054 clang::Parser::ParseBraceInitializer() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6af9054)
#50 0x0000000006a8a470 clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a8a470)
#51 0x0000000006a97e5a 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+++0x6a97e5a)
#52 0x0000000006a5555b clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a5555b)
#53 0x0000000006a55d4f clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a55d4f)
#54 0x0000000006a5db7a clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a5db7a)
#55 0x0000000006a5eb15 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a5eb15)
#56 0x0000000006a5efc0 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a5efc0)
#57 0x0000000006a3fb63 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a3fb63)
#58 0x0000000004c6daf8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c6daf8)
#59 0x0000000004f66ee5 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f66ee5)
#60 0x0000000004ee6d1e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ee6d1e)
#61 0x000000000506158d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x506158d)
#62 0x0000000000de8a9e cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde8a9e)
#63 0x0000000000ddf46a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#64 0x0000000000ddf5ed int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#65 0x0000000004ce0b39 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#66 0x000000000424aea4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x424aea4)
#67 0x0000000004ce114f clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#68 0x0000000004ca1ae2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ca1ae2)
#69 0x0000000004ca2a8e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ca2a8e)
#70 0x0000000004ca9ee5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ca9ee5)
#71 0x0000000000de4e11 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde4e11)
#72 0x0000000000c8f104 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc8f104)
#73 0x00007090d9829d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#74 0x00007090d9829e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#75 0x0000000000ddef05 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xddef05)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

Start by reproducing the assertion with the provided source and Compiler Explorer command. Read clang::Sema::ActOnCapScopeReturnStmt and clang::Sema::BuildReturnStmt, then inspect the QualType::getCommonPtr assertion in llvm/tools/clang/include/clang/AST/TypeBase.h; done means the reproducer no longer asserts and regression coverage is added.

Written by the indexing model from the issue text.