Add -strip-field-names and -strip-field-metadata IRGen options

This controls emission of field metadata for reflection, providing
the default decision. We might want to explore finer-grained
control per type, likely as a source code annotation.

  -strip-field-names
    Strip field names from nominal type metadata.

  -strip-field-metadata
    Strip all field metadata for nominal types. This also implies
    -strip-field-names.

NFC yet.

lib/Frontend/CompilerInvocation.cpp

@@ -1204,6 +1204,15 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     }
   }
 
+  if (Args.hasArg(OPT_strip_field_names)) {
+    Opts.StripFieldNames = true;
+  }
+
+  if (Args.hasArg(OPT_strip_field_metadata)) {
+    Opts.StripFieldMetadata = true;
+    Opts.StripFieldNames = true;
+  }
+
   return false;
 }