[Diagnostics] Add -suppress-notes flag

We already have -suppress-warnings and -suppress-remarks; this patch
adds support for suppressing notes too. Doing so is useful for -verify
tests where we don't really care about the emitted notes.

lib/Frontend/CompilerInvocation.cpp

@@ -2652,6 +2652,7 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
   }
 
   Opts.SuppressWarnings |= Args.hasArg(OPT_suppress_warnings);
+  Opts.SuppressNotes |= Args.hasArg(OPT_suppress_notes);
   Opts.SuppressRemarks |= Args.hasArg(OPT_suppress_remarks);
   for (const Arg *arg : Args.filtered(OPT_warning_treating_Group)) {
     Opts.WarningsAsErrorsRules.push_back([&] {
@@ -2732,6 +2733,9 @@ static void configureDiagnosticEngine(
   if (Options.SuppressWarnings) {
     Diagnostics.setSuppressWarnings(true);
   }
+  if (Options.SuppressNotes) {
+    Diagnostics.setSuppressNotes(true);
+  }
   if (Options.SuppressRemarks) {
     Diagnostics.setSuppressRemarks(true);
   }