[clang-check][Syntax] Assertion `SN != nullptr' failed in TreeBuilder::markChild when parsing explicit object parameter with variadic arguments

This input is generated by a fuzzer.

```cxx
void fn4(this S1& self, const char *out, ... )
{
va_list args;
self.print(out, args);
}
```
I cannot find clang-check on godbolt and I don't know if there is a similar approach to trigger this crash on clang++.

use

```
clang-check

--syntax-tree-dump
-- -xc++ -std=c++23
```

stacktrace
```
clang-check: /home/ubuntu2404/llvm-project-196067/clang/lib/Tooling/Syntax/BuildTree.cpp:1699: void clang::syntax::TreeBuilder::markChild({anonymous}::ASTPtr, clang::syntax::NodeRole): Assertion `SN != nullptr' failed.
#0 0x000057b374f2c1f1 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x96d1f1)
#1 0x000057b374f28dac llvm::sys::RunSignalHandlers() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x969dac)
#2 0x000057b374f28f0c SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#3 0x00007c6a5ea45330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#4 0x00007c6a5ea9eb2c __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
#5 0x00007c6a5ea9eb2c __pthread_kill_internal ./nptl/pthread_kill.c:78:10
#6 0x00007c6a5ea9eb2c pthread_kill ./nptl/pthread_kill.c:89:10
#7 0x00007c6a5ea4527e raise ./signal/../sysdeps/posix/raise.c:27:6
#8 0x00007c6a5ea288ff abort ./stdlib/abort.c:81:7
#9 0x00007c6a5ea2881b _nl_load_domain ./intl/loadmsgcat.c:1177:9
#10 0x00007c6a5ea3b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#11 0x000057b3752c4277 (anonymous namespace)::BuildTreeVisitor::WalkUpFromFunctionTypeLoc(clang::FunctionTypeLoc) BuildTree.cpp:0:0
#12 0x000057b3752cc028 clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseFunctionProtoTypeLoc(clang::FunctionProtoTypeLoc, bool) (.isra.0) BuildTree.cpp:0:0
#13 0x000057b3752cd0f5 clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseTypeLoc(clang::TypeLoc, bool) (.constprop.0) BuildTree.cpp:0:0
#14 0x000057b3752de59f clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseFunctionHelper(clang::FunctionDecl*) BuildTree.cpp:0:0
#15 0x000057b3752caddc clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseDecl(clang::Decl*) BuildTree.cpp:0:0
#16 0x000057b3752cbaea clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseDeclContextHelper(clang::DeclContext*) BuildTree.cpp:0:0
#17 0x000057b3752e483b clang::RecursiveASTVisitor<(anonymous namespace)::BuildTreeVisitor>::TraverseTranslationUnitDecl(clang::TranslationUnitDecl*) BuildTree.cpp:0:0
#18 0x000057b3752cb439 clang::syntax::buildSyntaxTree(clang::syntax::Arena&, clang::syntax::TokenBufferTokenManager&, clang::ASTContext&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xd0c439)
#19 0x000057b3749bf4af (anonymous namespace)::DumpSyntaxTree::CreateASTConsumer(clang::CompilerInstance&, llvm::StringRef)::Consumer::HandleTranslationUnit(clang::ASTContext&) ClangCheck.cpp:0:0
#20 0x000057b3767f5f9c clang::ParseAST(clang::Sema&, bool, bool) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x2236f9c)
#21 0x000057b374f83271 clang::FrontendAction::Execute() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x9c4271)
#22 0x000057b374fcf308 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xa10308)
#23 0x000057b3752abcb7 clang::tooling::FrontendActionFactory::runInvocation(std::shared_ptr, clang::FileManager*, std::shared_ptr, clang::DiagnosticConsumer*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceccb7)
#24 0x000057b3752a5b2a 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 0x000057b3752a81e2 clang::tooling::ToolInvocation::run() (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xce91e2)
#26 0x000057b3752a9e19 clang::tooling::ClangTool::run(clang::tooling::ToolAction*) (/home/ubuntu2404/build-196067-assert/bin/clang-check+0xceae19)
#27 0x000057b374944405 main (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x385405)
#28 0x00007c6a5ea2a1ca __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#29 0x00007c6a5ea2a28b call_init ./csu/../csu/libc-start.c:128:20
#30 0x00007c6a5ea2a28b __libc_start_main ./csu/../csu/libc-start.c:347:5
#31 0x000057b3749be045 _start (/home/ubuntu2404/build-196067-assert/bin/clang-check+0x3ff045)
Aborted (core dumped)
```

Start by running clang-check with --syntax-tree-dump on the supplied C++23 reproducer and read clang/lib/Tooling/Syntax/BuildTree.cpp around TreeBuilder::markChild and WalkUpFromFunctionTypeLoc. Trace the explicit object parameter with variadic arguments, then add a regression test if the surrounding tests identify an appropriate location. Done means the reproducer no longer triggers the assertion and syntax-tree dumping completes.

Written by the indexing model from the issue text.