[flang] Remove the strict F18 checking flag. Always check that program is

strictly conforming to F18, but demote errors to warnings when the
Fortran is legal under pre-F18 standards. If the Fortran is illegal
under F18 _and_ previous standards, then emit an error.

Original-commit: flang-compiler/f18@6d1f37683a4409a609af49c4a623007950c868fc
Reviewed-on: https://github.com/flang-compiler/f18/pull/170
Tree-same-pre-rewrite: false