Add an option -fmodules-validate-system-headers

When enabled, always validate the system headers when loading a module.
The end result of this is that when these headers change, we will notice
and rebuild the module.

llvm-svn: 203630

clang/lib/Frontend/CompilerInvocation.cpp

@@ -930,6 +930,9 @@ static void ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args) {
       Args.hasArg(OPT_fmodules_validate_once_per_build_session);
   Opts.BuildSessionTimestamp =
       getLastArgUInt64Value(Args, OPT_fbuild_session_timestamp, 0);
+  Opts.ModulesValidateSystemHeaders =
+      Args.hasArg(OPT_fmodules_validate_system_headers);
+
   for (arg_iterator it = Args.filtered_begin(OPT_fmodules_ignore_macro),
                     ie = Args.filtered_end();
        it != ie; ++it) {