[flang] Version information in flang/f18

[flang] Version information in flang/f18

Fixed some version information in flang/f18:

  - fixed the behavior of the -v switch: this flag enables verbosity with used with arguments, but just displays the version when used alone (related to this bug: https://bugs.llvm.org/show_bug.cgi?id=46017)
 - added __FLANG, __FLANG_MAJOR__, __FLANG_MINOR__ and __FLANG_PATCHLEVEL__ (similar to their __F18* counterparts) for compatibility purpose

Reviewed By: sscalpone, AlexisPerry, richard.barton.arm, tskeith

Differential Revision: https://reviews.llvm.org/D84334