[clang-check][syntax]Crash in ASTToSyntaxMapping::add: `Added && "mapping added twice"'` when handling DecompositionDecl

This input is generated by a fuzzer.
```cxx
auto [bar] = foo;
```

```
clang-check --tokens-dump example.cpp -- -xc++ -std=c++23
```

```
clang-check: /home/ubuntu2404/llvm-project-196067/clang/lib/Tooling/Syntax/BuildTree.cpp:324: void {anonymous}::ASTToSyntaxMapping::add({anonymous}::ASTPtr, clang::syntax::Tree*): Assertion `Added && "mapping added twice"' failed.
#0 0x00006069a803d1f1 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x96d1f1)
#1 0x00006069a8039dac llvm::sys::RunSignalHandlers() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x969dac)
#2 0x00006069a8039f0c SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#3 0x000071a12ee45330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#4 0x000071a12ee9eb2c __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
#5 0x000071a12ee9eb2c __pthread_kill_internal ./nptl/pthread_kill.c:78:10
#6 0x000071a12ee9eb2c pthread_kill ./nptl/pthread_kill.c:89:10
#7 0x000071a12ee4527e raise ./signal/../sysdeps/posix/raise.c:27:6
#8 0x000071a12ee288ff abort ./stdlib/abort.c:81:7
#9 0x000071a12ee2881b _nl_load_domain ./intl/loadmsgcat.c:1177:9
#10 0x000071a12ee3b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#11 0x00006069a83ca045 (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xcfa045)
#12 0x00006069a83d4d9a clang::syntax::TreeBuilder::foldNode(llvm::ArrayRef, clang::syntax::Tree*, llvm::PointerUnion) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xd04d9a)
#13 0x00006069a83d884b (anonymous namespace)::BuildTreeVisitor::VisitDecl(clang::Decl*) BuildTree.cpp:0:0
#14 0x00006069a83f5b84 clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseDecompositionDecl(clang::DecompositionDecl*) BuildTree.cpp:0:0
#15 0x00006069a83dcaea clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseDeclContextHelper(clang::DeclContext*) BuildTree.cpp:0:0
#16 0x00006069a83f583b clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseTranslationUnitDecl(clang::TranslationUnitDecl*) BuildTree.cpp:0:0
#17 0x00006069a83dc439 clang::syntax::buildSyntaxTree(clang::syntax::Arena&, clang::syntax::TokenBufferTokenManager&, clang::ASTContext&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xd0c439)
#18 0x00006069a7ad04af (anonymous namespace)::DumpSyntaxTree::CreateASTConsumer(clang::CompilerInstance&, llvm::StringRef)::Consumer::HandleTranslationUnit(clang::ASTContext&) ClangCheck.cpp:0:0
#19 0x00006069a9906f9c clang::ParseAST(clang::Sema&, bool, bool) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x2236f9c)
#20 0x00006069a8094271 clang::FrontendAction::Execute() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x9c4271)
#21 0x00006069a80e0308 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xa10308)
#22 0x00006069a83bccb7 clang::tooling::FrontendActionFactory::runInvocation(std::shared_ptr, clang::FileManager*, std::shared_ptr, clang::DiagnosticConsumer*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceccb7)
#23 0x00006069a83b6b2a clang::tooling::ToolInvocation::runInvocation(char const*, clang::driver::Compilation*, std::shared_ptr, std::shared_ptr) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xce6b2a)
#24 0x00006069a83b91e2 clang::tooling::ToolInvocation::run() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xce91e2)
#25 0x00006069a83bae19 clang::tooling::ClangTool::run(clang::tooling::ToolAction*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceae19)
#26 0x00006069a7a55405 main (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x385405)
#27 0x000071a12ee2a1ca __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#28 0x000071a12ee2a28b call_init ./csu/../csu/libc-start.c:128:20
#29 0x000071a12ee2a28b __libc_start_main ./csu/../csu/libc-start.c:347:5
#30 0x00006069a7acf045 _start (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x3ff045)
Aborted (core dumped)
```

Reproduce the crash with clang-check --tokens-dump example.cpp -- -xc++ -std=c++23 using the shown decomposition declaration. Start in clang/lib/Tooling/Syntax/BuildTree.cpp at ASTToSyntaxMapping::add, then trace TreeBuilder::foldNode and BuildTreeVisitor's DecompositionDecl traversal. Done means this input no longer triggers the mapping-added-twice assertion.

Written by the indexing model from the issue text.