[flang] Ensure Preprocessor::Define() saves macro names correctly

[klausler]

Summary

This fixes problems with macros defined with -D on the command line
and predefined macros defined in the throwaway driver program.

[PeteSteinfeld]

All builds, tests, and looks good.

[DavidTruby]

LGTM! It would be great to have test so this doesn't regress

flang/lib/Parser/preprocessor.cpp

 }
 
 void Preprocessor::Define(std::string macro, std::string value) {
-  definitions_.emplace(macro, Definition{value, allSources_});
+  definitions_.emplace(SaveTokenAsName(macro), Definition{value, allSources_});
 }
 
 void Preprocessor::Undefine(std::string macro) { definitions_.erase(macro); }