[clang] crashes on structured binding with incomplete declaration

Reproducer:
https://godbolt.org/z/hKzcGjK6e
```cpp
auto[a,
```

Backtrace:
```console
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
1. parser at end of file
#0 0x00000000043f9708 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f9708)
#1 0x00000000043f65c4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f65c4)
#2 0x00000000043f6bec llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f6bec)
#3 0x0000000004335c28 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x0000768623045330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x000076862309eb2c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb2c)
#6 0x000076862304527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007686230288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x0000000000d9dac2 (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd9dac2)
#9 0x00000000070f14c6 clang::Sema::ActOnUninitializedDecl(clang::Decl*) (.part.0) SemaDecl.cpp:0:0
#10 0x0000000006d3f99b clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d3f99b)
#11 0x0000000006d60529 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+++0x6d60529)
#12 0x0000000006d0bafb clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d0bafb)
#13 0x0000000006d0c2ef clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d0c2ef)
#14 0x0000000006d18b01 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d18b01)
#15 0x0000000006d19a15 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d19a15)
#16 0x0000000006d19e00 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d19e00)
#17 0x0000000006cf68f3 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cf68f3)
#18 0x0000000004ef7c58 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ef7c58)
#19 0x00000000052234b8 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52234b8)
#20 0x00000000051b0abd clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51b0abd)
#21 0x00000000052fb3cd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52fb3cd)
#22 0x0000000000da6cbc cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda6cbc)
#23 0x0000000000d9d8ca ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#24 0x0000000000d9da4d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#25 0x0000000004f6ef29 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#26 0x0000000004336064 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4336064)
#27 0x0000000004f6fa95 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#28 0x0000000004f28462 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f28462)
#29 0x0000000004f2cc97 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f2cc97)
#30 0x0000000004f371f5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f371f5)
#31 0x0000000000da31aa clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda31aa)
#32 0x0000000000cbee3a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcbee3a)
#33 0x000076862302a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#34 0x000076862302a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#35 0x0000000000d9d275 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd9d275)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```

Start with the Godbolt reproducer and inspect clang::Sema::ActOnUninitializedDecl in SemaDecl.cpp, along with the parser entry points shown in the backtrace. Reproduce the crash locally, then add coverage for the incomplete structured binding and verify that Clang reports an error instead of aborting.

Written by the indexing model from the issue text.