[clang-check][Syntax] Crash in `TreeBuilder::getRange: assertion (BeginChildren == Trees.end() || BeginChildren->first == FirstToken) && "fold crosses boundaries of existing subtrees` fails when dumping tokens for malformed C code

``` c
foo(bar)
char foo = {};};

```
use

```
clang-check \
--tokens-dump \
example.c \
-- -xc -std=c11
```

stacktrace
```
clang-check: /home/ubuntu2404/llvm-project-196067/clang/lib/Tooling/Syntax/BuildTree.cpp:632: void clang::syntax::TreeBuilder::Forest::foldChildren(const clang::syntax::TokenBuffer&, llvm::ArrayRef, clang::syntax::Tree*): Assertion `(BeginChildren == Trees.end() || BeginChildren->first == FirstToken) && "fold crosses boundaries of existing subtrees"' failed.
#0 0x000064e13d96f1f1 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x96d1f1)
#1 0x000064e13d96bdac llvm::sys::RunSignalHandlers() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x969dac)
#2 0x000064e13d96bf0c SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#3 0x0000777cbbe45330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#4 0x0000777cbbe9eb2c __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
#5 0x0000777cbbe9eb2c __pthread_kill_internal ./nptl/pthread_kill.c:78:10
#6 0x0000777cbbe9eb2c pthread_kill ./nptl/pthread_kill.c:89:10
#7 0x0000777cbbe4527e raise ./signal/../sysdeps/posix/raise.c:27:6
#8 0x0000777cbbe288ff abort ./stdlib/abort.c:81:7
#9 0x0000777cbbe2881b _nl_load_domain ./intl/loadmsgcat.c:1177:9
#10 0x0000777cbbe3b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#11 0x000064e13dd06ecb clang::syntax::TreeBuilder::foldNode(llvm::ArrayRef, clang::syntax::Tree*, llvm::PointerUnion) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xd04ecb)
#12 0x000064e13dd26693 clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseEmptyDecl(clang::EmptyDecl*) BuildTree.cpp:0:0
#13 0x000064e13dd0eaea clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseDeclContextHelper(clang::DeclContext*) BuildTree.cpp:0:0
#14 0x000064e13dd2783b clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseTranslationUnitDecl(clang::TranslationUnitDecl*) BuildTree.cpp:0:0
#15 0x000064e13dd0e439 clang::syntax::buildSyntaxTree(clang::syntax::Arena&, clang::syntax::TokenBufferTokenManager&, clang::ASTContext&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xd0c439)
#16 0x000064e13d4024af (anonymous namespace)::DumpSyntaxTree::CreateASTConsumer(clang::CompilerInstance&, llvm::StringRef)::Consumer::HandleTranslationUnit(clang::ASTContext&) ClangCheck.cpp:0:0
#17 0x000064e13f238f9c clang::ParseAST(clang::Sema&, bool, bool) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x2236f9c)
#18 0x000064e13d9c6271 clang::FrontendAction::Execute() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x9c4271)
#19 0x000064e13da12308 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xa10308)
#20 0x000064e13dceecb7 clang::tooling::FrontendActionFactory::runInvocation(std::shared_ptr, clang::FileManager*, std::shared_ptr, clang::DiagnosticConsumer*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceccb7)
#21 0x000064e13dce8b2a clang::tooling::ToolInvocation::runInvocation(char const*, clang::driver::Compilation*, std::shared_ptr, std::shared_ptr) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xce6b2a)
#22 0x000064e13dceb1e2 clang::tooling::ToolInvocation::run() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xce91e2)
#23 0x000064e13dcece19 clang::tooling::ClangTool::run(clang::tooling::ToolAction*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceae19)
#24 0x000064e13d387405 main (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x385405)
#25 0x0000777cbbe2a1ca __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#26 0x0000777cbbe2a28b call_init ./csu/../csu/libc-start.c:128:20
#27 0x0000777cbbe2a28b __libc_start_main ./csu/../csu/libc-start.c:347:5
#28 0x000064e13d401045 _start (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x3ff045)
Aborted (core dumped)
```

Start by reproducing the crash with clang-check --tokens-dump on the provided example.c using -xc -std=c11. Read clang/lib/Tooling/Syntax/BuildTree.cpp around TreeBuilder::Forest::foldChildren and foldNode, then add coverage for the malformed input and confirm token dumping no longer hits the assertion.

Written by the indexing model from the issue text.