Conditionally parse documentation comments in system headers by
passing -fretain-comments-from-system-headers.  By default, the
compiler no longer parses such documentation comments, as they
can result in a noticeable compile time/PCH slowdown.

Fixes <rdar://problem/11860820>.

llvm-svn: 163778
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 6d6dbfc..6d34425 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2151,6 +2151,9 @@
 
   Opts.EmitMicrosoftInlineAsm = Args.hasArg(OPT_fenable_experimental_ms_inline_asm);
 
+  Opts.RetainCommentsFromSystemHeaders =
+      Args.hasArg(OPT_fretain_comments_from_system_headers);
+
   unsigned SSP = Args.getLastArgIntValue(OPT_stack_protector, 0, Diags);
   switch (SSP) {
   default: