[clangd] Set RetainCommentsFromSystemHeaders to true

[lh123]

Summary

clangd should retain comments from system headers.

fixes https://github.com/clangd/clangd/issues/96

note: I don't know much about how to write unit tests, but it works fine on my computer.

[ilya-biryukov]

LGTM, thanks!
Do you need someone to land this?

[lh123]

In D69928#1736569, @ilya-biryukov wrote:

LGTM, thanks!
Do you need someone to land this?

Yes, I can‘t commit it.

clang-tools-extra/clangd/Compiler.cpp

buildCompilerInvocation(const ParseInputs &Inputs,
   std::unique_ptr<CompilerInvocation> CI = createInvocationFromCommandLine(
       ArgStrs, CommandLineDiagsEngine, Inputs.FS,
       /*ShouldRecoverOnErrors=*/true);
   if (!CI)
     return nullptr;
   // createInvocationFromCommandLine sets DisableFree.
   CI->getFrontendOpts().DisableFree = false;
   CI->getLangOpts()->CommentOpts.ParseAllComments = true;
+  CI->getLangOpts()->RetainCommentsFromSystemHeaders = true;
   return CI;
 }
 
 std::unique_ptr<CompilerInstance>
 prepareCompilerInstance(std::unique_ptr<clang::CompilerInvocation> CI,
                         const PrecompiledPreamble *Preamble,
                         std::unique_ptr<llvm::MemoryBuffer> Buffer,
                         llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,

clang-tools-extra/clangd/index/IndexAction.cpp

return index::createIndexingASTConsumer(
             return false;
           return !Collector->shouldIndexFile(FID);
         });
   }
 
   bool BeginInvocation(CompilerInstance &CI) override {
     // We want all comments, not just the doxygen ones.
     CI.getLangOpts().CommentOpts.ParseAllComments = true;
+    CI.getLangOpts().RetainCommentsFromSystemHeaders = true;
     // Index the whole file even if there are warnings and -Werror is set.
     // Avoids some analyses too. Set in two places as we're late to the party.
     CI.getDiagnosticOpts().IgnoreWarnings = true;
     CI.getDiagnostics().setIgnoreAllWarnings(true);
     // Instruct the parser to ask our ASTConsumer if it should skip function
     // bodies. The ASTConsumer will take care of skipping only functions inside
     // the files that we have already processed.
     CI.getFrontendOpts().SkipFunctionBodies = true;