[IRGen] Added gate for metadata prespecialization.

The new frontend flag -prespecialize-generic-metadata must be passed in
order for generic metadata to be specialized statically.

rdar://problem/56984885

lib/Frontend/CompilerInvocation.cpp

@@ -1249,6 +1249,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.DisableLegacyTypeInfo = true;
   }
 
+  if (Args.hasArg(OPT_prespecialize_generic_metadata)) {
+    Opts.PrespecializeGenericMetadata = true;
+  }
+
   if (const Arg *A = Args.getLastArg(OPT_read_legacy_type_info_path_EQ)) {
     Opts.ReadLegacyTypeInfoPath = A->getValue();
   }