[flang][driver] Add standard macro predefinitions for compiler version

Add the following standard predefinitions that f18 supports:
  * `__flang__`,
  * `__flang_major__`,
  * `__flang_minor__`,
  * `__flang_patchlevel__`

Summary of changes:
- Populate Fortran::parser::Options#predefinitions with the default
  supported predefinitions

Differential Revision: https://reviews.llvm.org/D94516