[flang] Allow ! and // comments after some preprocessing directives

Old-style C /*comments*/ are omitted from preprocessor directive
token sequences by the prescanner, but line-ending C++ and Fortran
free-form comments are not since their handling might depend on
the directive.  Add code to skip these line-ending comments as
appropriate in place of existing code that just skipped blanks.

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D84061