[flang] Make prescanner tokenization message re: Hollerith truncation a...

[flang] Make prescanner tokenization message re: Hollerith truncation a warning, so parser get a shot at it.  Merge messages from failed parsing alternatives more informatively.

Original-commit: flang-compiler/f18@16ed01d8f372c6c64341dd793b454e37581fa83d
Reviewed-on: https://github.com/flang-compiler/f18/pull/161