[clangd] Set RetainCommentsFromSystemHeaders to true

clangd should retain comments from system headers.

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

Patch by lh123!

Differential revision: https://reviews.llvm.org/D69928

clang-tools-extra/clangd/Compiler.cpp

@@ -63,6 +63,7 @@ buildCompilerInvocation(const ParseInputs &Inputs,
   // createInvocationFromCommandLine sets DisableFree.
   CI->getFrontendOpts().DisableFree = false;
   CI->getLangOpts()->CommentOpts.ParseAllComments = true;
+  CI->getLangOpts()->RetainCommentsFromSystemHeaders = true;
   return CI;
 }
 

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

@@ -160,6 +160,7 @@ public:
   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;