[flang] Accept more unrecognized !DIR$ compiler directives (#85829)

When encountering an unparsable !DIR$ compiler directive line, accept it
as a whole source line and emit a warning that it is unrecognizable.

Fixes https://github.com/llvm/llvm-project/issues/59107,
https://github.com/llvm/llvm-project/issues/82212, and
https://github.com/llvm/llvm-project/issues/82654.