[flang] Accept directive sentinels in macro-replaced source better (#70699)

At present, the prescanner emits an error if a source line or compiler
directive, after macro replacement or not, contains a token with a
non-Fortran character. In the particular case of the '!' character, the
code that checks for bad character will accept the '!' if it appears
after a ';', since the '!' might begin a compiler directive.

This current implementation fails when a compiler directive appears
after some other character that might (by means of further source
processing not visible to the prescanner) be replaced with a ';' or
newline.

Extend the bad character check for '!' to actually check for a compiler
directive sentinel instead.