Complain about attempts to use 'protected' visibility on targets

like Darwin that don't support it.  We should also complain about
invalid -fvisibility=protected, but that information doesn't seem
to exist at the most appropriate time, so I've left a FIXME behind.

llvm-svn: 149186

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1792,6 +1792,7 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   else if (Vis == "hidden")
     Opts.setVisibilityMode(HiddenVisibility);
   else if (Vis == "protected")
+    // FIXME: diagnose if target does not support protected visibility
     Opts.setVisibilityMode(ProtectedVisibility);
   else
     Diags.Report(diag::err_drv_invalid_value)