[clang-tools-extra] [clangd] [C++ Modules] Skip PCH when TU imports modules (PR #187432)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

> I didn't update that patch. I observed a similar crash and I didn't meet it after I use this patch. That crash starts with TUScheduler's UpdatePreamble and then calls into ParsedAST::build.

For me the trace is the following and it looks like that this patch can't prevent this crash (but I applied this patch just to be sure and behavior is the same).
```
 #0 0x000055ef32d0d0e0 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (.../llvm-project/build_clang/bin/clangd+0x52a0e0)
 #1 0x000055ef32d09f6d SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #2 0x00007fee7d0fb520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #3 0x000055ef3552c821 clang::ASTReader::FindExternalLexicalDecls(clang::DeclContext const*, llvm::function_ref<bool (clang::Decl::Kind)>, llvm::SmallVectorImpl<clang::Decl*>&) (.../llvm-project/build_clang/bin/clangd+0x2d49821)
 #4 0x000055ef32e69ac9 clang::DeclContext::LoadLexicalDeclsFromExternalStorage() const (.../llvm-project/build_clang/bin/clangd+0x686ac9)
 #5 0x000055ef32e69c4d clang::DeclContext::decls_begin() const (.../llvm-project/build_clang/bin/clangd+0x686c4d)
 #6 0x000055ef345dcf49 clang::clangd::indexHeaderSymbols(llvm::StringRef, clang::ASTContext&, clang::Preprocessor&, clang::include_cleaner::PragmaIncludes const&, clang::clangd::SymbolOrigin) (.../llvm-project/build_clang/bin/clangd+0x1df9f49)
 #7 0x000055ef345dd2ad clang::clangd::FileIndex::updatePreamble(llvm::StringRef, llvm::StringRef, clang::ASTContext&, clang::Preprocessor&, clang::include_cleaner::PragmaIncludes const&) (.../llvm-project/build_clang/bin/clangd+0x1dfa2ad)
 #8 0x000055ef3426b18c void llvm::detail::UniqueFunctionBase<void>::CallImpl<clang::clangd::(anonymous namespace)::UpdateIndexCallbacks::onPreambleAST(llvm::StringRef, llvm::StringRef, clang::clangd::CapturedASTCtx, std::shared_ptr<clang::include_cleaner::PragmaIncludes const>)::'lambda'()>(void*) ClangdServer.cpp:0:0
 #9 0x000055ef34675a06 void* llvm::thread::ThreadProxy<std::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::'lambda0'()>>(void*) Threading.cpp:0:0
#10 0x00007fee7d14dac3 start_thread ./nptl/pthread_create.c:442:8
#11 0x00007fee7d1df8d0 ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:83:0
```

> I think if you really want that, you can make a patch and let's review it.

I plan to check how difficult it will be to implement this at the end of this week if no one does this first

https://github.com/llvm/llvm-project/pull/187432