[macros] add -Rmacro-expansions

This adds the -Rmacro-expansions flag. It provides similar functionality
to -dump-macro-expansions, but instead of dumping the macro expansion to
stderr, it emits it line by line as remarks. This is useful for testing
with -verify, where both macro expansion content and warnings need to be
tested at the same time.

lib/Frontend/CompilerInvocation.cpp

@@ -1774,6 +1774,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DumpMacroExpansions = Args.hasArg(
       OPT_dump_macro_expansions);
 
+  Opts.RemarkMacroExpansions = Args.hasArg(
+      OPT_expansion_remarks);
+
   Opts.DumpSourceFileImports = Args.hasArg(
       OPT_dump_source_file_imports);