[flang] Minor speed-up to module file parsing (#152178)

Module files shouldn't ever produce parsing errors, and if they did in
the case of a badly-generated module file, the compiler will notice and
crash. So we can run the parser on module files with message deferral
enabled, and that saves time that would otherwise be spent generating
messages on failed parsing alternatives that are discarded anyway when
backtracking. It's not a big savings (single digit percentage on overall
compilation time for a big application with lots of modules), but worth
doing.

Author: klausler

flang/lib/Parser/parsing.cpp

@@ -285,6 +285,8 @@ void Parsing::Parse(llvm::raw_ostream &out) {
       .set_log(&log_);
   ParseState parseState{cooked()};
   parseState.set_inFixedForm(options_.isFixedForm).set_userState(&userState);
+  // Don't bother managing message buffers when parsing module files.
+  parseState.set_deferMessages(options_.isModuleFile);
   parseTree_ = program.Parse(parseState);
   CHECK(
       !parseState.anyErrorRecovery() || parseState.messages().AnyFatalError());