[clang-check][syntax]Crash in TreeBuilder::assignRole: `It != Trees.end() && "no node found" ` for malformed C++ function body

This input is generated by a fuzzer.
```c
a() {
```

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

```
clang-check: /home/ubuntu2404/llvm-project-196067/clang/lib/Tooling/Syntax/BuildTree.cpp:584: void clang::syntax::TreeBuilder::Forest::assignRole(llvm::ArrayRef, clang::syntax::NodeRole): Assertion `It != Trees.end() && "no node found"' failed.
#0 0x0000634e236aa1f1 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x96d1f1)
#1 0x0000634e236a6dac llvm::sys::RunSignalHandlers() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x969dac)
#2 0x0000634e236a6f0c SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#3 0x00007546dd445330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#4 0x00007546dd49eb2c __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
#5 0x00007546dd49eb2c __pthread_kill_internal ./nptl/pthread_kill.c:78:10
#6 0x00007546dd49eb2c pthread_kill ./nptl/pthread_kill.c:89:10
#7 0x00007546dd44527e raise ./signal/../sysdeps/posix/raise.c:27:6
#8 0x00007546dd4288ff abort ./stdlib/abort.c:81:7
#9 0x00007546dd42881b _nl_load_domain ./intl/loadmsgcat.c:1177:9
#10 0x00007546dd43b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#11 0x0000634e23a3edf6 clang::syntax::TreeBuilder::markChildToken(clang::SourceLocation, clang::syntax::NodeRole) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xd01df6)
#12 0x0000634e23a455dd (anonymous namespace)::BuildTreeVisitor::WalkUpFromCompoundStmt(clang::CompoundStmt*) BuildTree.cpp:0:0
#13 0x0000634e23a51aae clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseStmt(clang::Stmt*, llvm::SmallVectorImpl, llvm::PointerIntPairInfo>>>*) (.constprop.0) BuildTree.cpp:0:0
#14 0x0000634e23a5c740 clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseFunctionHelper(clang::FunctionDecl*) BuildTree.cpp:0:0
#15 0x0000634e23a48ddc clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseDecl(clang::Decl*) BuildTree.cpp:0:0
#16 0x0000634e23a49aea clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseDeclContextHelper(clang::DeclContext*) BuildTree.cpp:0:0
#17 0x0000634e23a6283b clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseTranslationUnitDecl(clang::TranslationUnitDecl*) BuildTree.cpp:0:0
#18 0x0000634e23a49439 clang::syntax::buildSyntaxTree(clang::syntax::Arena&, clang::syntax::TokenBufferTokenManager&, clang::ASTContext&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xd0c439)
#19 0x0000634e2313d4af (anonymous namespace)::DumpSyntaxTree::CreateASTConsumer(clang::CompilerInstance&, llvm::StringRef)::Consumer::HandleTranslationUnit(clang::ASTContext&) ClangCheck.cpp:0:0
#20 0x0000634e24f73f9c clang::ParseAST(clang::Sema&, bool, bool) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x2236f9c)
#21 0x0000634e23701271 clang::FrontendAction::Execute() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x9c4271)
#22 0x0000634e2374d308 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xa10308)
#23 0x0000634e23a29cb7 clang::tooling::FrontendActionFactory::runInvocation(std::shared_ptr, clang::FileManager*, std::shared_ptr, clang::DiagnosticConsumer*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceccb7)
#24 0x0000634e23a23b2a clang::tooling::ToolInvocation::runInvocation(char const*, clang::driver::Compilation*, std::shared_ptr, std::shared_ptr) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xce6b2a)
#25 0x0000634e23a261e2 clang::tooling::ToolInvocation::run() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xce91e2)
#26 0x0000634e23a27e19 clang::tooling::ClangTool::run(clang::tooling::ToolAction*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceae19)
#27 0x0000634e230c2405 main (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x385405)
#28 0x00007546dd42a1ca __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#29 0x00007546dd42a28b call_init ./csu/../csu/libc-start.c:128:20
#30 0x00007546dd42a28b __libc_start_main ./csu/../csu/libc-start.c:347:5
#31 0x0000634e2313c045 _start (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x3ff045)
Aborted (core dumped)
```

Reproduce the crash with the shown malformed input and clang-check command, then inspect clang/lib/Tooling/Syntax/BuildTree.cpp around assignRole, markChildToken, and the CompoundStmt traversal in the stack trace. Trace how the incomplete function body is represented and add coverage using the reported input. Done means clang-check no longer hits the assertion while processing this file.

Written by the indexing model from the issue text.