[flang][driver] Report prescanning diags during syntax-only parsing

Ensure diagnostics from the prescanner are reported when running `flang-new -fsyntax-only` (i.e. only syntax parsing).
This keeps the diagnostics output of flang-new consistent with `f18 -fparse-only` when running the syntax parsing action, ParseSyntaxOnlyAction.

Summary of changes:
- Modify ParseSyntaxOnlyAction::ExecuteAction to report diagnostics

Differential Revision: https://reviews.llvm.org/D95220