[flang] Rename DoChecker to DoForallChecker

This reflects the fact that it now performs checks on both DO
and FORALL constructs. Rename the source files as well.

Original-commit: flang-compiler/f18@8ecb6a279f53ff4cc9493506370664fb1923470a
Reviewed-on: https://github.com/flang-compiler/f18/pull/989