[Clang] Assertion failure in Sema::LookupTemplateName on invalid expression with trailing return type and template keyword

Code:
```CPP
a( (enum b )b { }
-> template c
```

Assertion failure:
```text
clang++: /root/llvm-project/llvm/tools/clang/lib/Sema/SemaTemplate.cpp:409: bool clang::Sema::LookupTemplateName(clang::LookupResult&, clang::Scope*, clang::CXXScopeSpec&, clang::QualType, bool, clang::Sema::RequiredTemplateKind, clang::Sema::AssumedTemplateKind*, bool): Assertion `(IsDependent || !ObjectType->isIncompleteType() || !ObjectType->getAs() || ObjectType->castAs()->getDecl()->isEntityBeingDefined()) && "Caller should have completed object type"' failed.
```

Backtrace:
```text
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. parser at end of file
#0 0x0000000004445bb8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4445bb8)
#1 0x00000000044429e4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44429e4)
#2 0x0000000004443004 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4443004)
#3 0x0000000004382328 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x0000731ad6e42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#5 0x0000731ad6e969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#6 0x0000731ad6e42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#7 0x0000731ad6e287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#8 0x0000731ad6e2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#9 0x0000731ad6e39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#10 0x000000000763e98f clang::Sema::LookupTemplateName(clang::LookupResult&, clang::Scope*, clang::CXXScopeSpec&, clang::QualType, bool, clang::Sema::RequiredTemplateKind, clang::Sema::AssumedTemplateKind*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x763e98f)
#11 0x000000000763feac clang::Sema::isTemplateName(clang::Scope*, clang::CXXScopeSpec&, bool, clang::UnqualifiedId const&, clang::OpaquePtr, bool, clang::OpaquePtr&, bool&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x763feac)
#12 0x00000000076404f9 clang::Sema::ActOnTemplateName(clang::Scope*, clang::CXXScopeSpec&, clang::SourceLocation, clang::UnqualifiedId const&, clang::OpaquePtr, bool, clang::OpaquePtr&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x76404f9)
#13 0x0000000006cbb0b1 clang::Parser::ParseUnqualifiedId(clang::CXXScopeSpec&, clang::OpaquePtr, bool, bool, bool, bool, bool, clang::SourceLocation*, clang::UnqualifiedId&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cbb0b1)
#14 0x0000000006ca7034 clang::Parser::ParsePostfixExpressionSuffix(clang::ActionResult) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ca7034)
#15 0x0000000006c9fd2a clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c9fd2a)
#16 0x0000000006c9ff9e clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c9ff9e)
#17 0x0000000006ca1fc7 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ca1fc7)
#18 0x0000000006caae18 clang::Parser::ParseParenExpression(clang::ParenParseOption&, bool, clang::ParenExprKind, clang::TypoCorrectionTypeBehavior, clang::OpaquePtr&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6caae18)
#19 0x0000000006ca0de0 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ca0de0)
#20 0x0000000006ca1fc7 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ca1fc7)
#21 0x0000000006ca2059 clang::Parser::ParseAssignmentExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ca2059)
#22 0x0000000006ca23be clang::Parser::ParseExpressionList(llvm::SmallVectorImpl&, llvm::function_ref, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ca23be)
#23 0x0000000006c5b6da clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c5b6da)
#24 0x0000000006c691aa 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+++0x6c691aa)
#25 0x0000000006c26efb clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c26efb)
#26 0x0000000006c276ff clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c276ff)
#27 0x0000000006c2ed21 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c2ed21)
#28 0x0000000006c2fc55 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c2fc55)
#29 0x0000000006c30040 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c30040)
#30 0x0000000006c112e3 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c112e3)
#31 0x0000000004f21698 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f21698)
#32 0x000000000522a7a5 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x522a7a5)
#33 0x00000000051a8eee clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51a8eee)
#34 0x000000000532711d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x532711d)
#35 0x0000000000de5fe2 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde5fe2)
#36 0x0000000000ddc91a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#37 0x0000000000ddca9d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#38 0x0000000004f96da9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#39 0x0000000004382764 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4382764)
#40 0x0000000004f973d6 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#41 0x0000000004f57032 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f57032)
#42 0x0000000004f57fde clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f57fde)
#43 0x0000000004f5fd25 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f5fd25)
#44 0x0000000000de2344 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde2344)
#45 0x0000000000c997ea main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc997ea)
#46 0x0000731ad6e29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#47 0x0000731ad6e29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#48 0x0000000000ddc3b5 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xddc3b5)
```

Goes back to Clang-12

See Compiler Explorer: [https://godbolt.org/z/9boocqjY8](https://godbolt.org/z/9boocqjY8)

The test case was generated by a fuzzer.