Add -fuse-line-directive flag to control usage of #line with -E

Currently -fms-extensions controls this behavior, which doesn't make
much sense. It means we can't identify what is and isn't a system header
when compiling our own preprocessed output, because #line doesn't
represent this information.

If someone is feeding Clang's preprocessed output to another compiler,
they can use this flag.

Fixes PR20553.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D5217

llvm-svn: 230587

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1826,6 +1826,7 @@ static void ParsePreprocessorOutputArgs(PreprocessorOutputOptions &Opts,
   Opts.ShowMacroComments = Args.hasArg(OPT_CC);
   Opts.ShowMacros = Args.hasArg(OPT_dM) || Args.hasArg(OPT_dD);
   Opts.RewriteIncludes = Args.hasArg(OPT_frewrite_includes);
+  Opts.UseLineDirectives = Args.hasArg(OPT_fuse_line_directives);
 }
 
 static void ParseTargetArgs(TargetOptions &Opts, ArgList &Args) {