[flang][driver] Make the driver report diagnostics from the prescanner

This patch makes sure that diagnostics from the prescanner are reported
when running `flang-new -E` (i.e. only the preprocessor phase is
requested). More specifically, the `PrintPreprocessedAction` action is
updated.

With this patch we make sure that the `f18` and `flang-new` provide
identical output when running the preprocessor and the prescanner
generates diagnostics.

Differential Revision: https://reviews.llvm.org/D94782