Sema: introduce a language option flag, DiagnosticsEditorMode. (#7982)

SourceKit always sets it positively. This may lead to more aggressive fixits however
less informative messages. We currently use the flag only for filling protocol stubs.

lib/Frontend/CompilerInvocation.cpp

@@ -837,6 +837,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.UseMalloc |= Args.hasArg(OPT_use_malloc);
 
+  Opts.DiagnosticsEditorMode |= Args.hasArg(OPT_diagnostics_editor_mode);
+
   Opts.EnableExperimentalPropertyBehaviors |=
     Args.hasArg(OPT_enable_experimental_property_behaviors);