[flang] Responses to comments on the pull request:

 - I removed the redundant test s3() from dosemantics90.f90
 - I changed the error messages to state "LOCAL locality-spec" rather than just
   "locality-spec"
 - I changed the names of a couple of variables/parameters in check-do.cc to
   make the code more understandable.

Original-commit: flang-compiler/f18@bcc6291e8370d97cb3d4c23a30e676bf3a0012ab
Reviewed-on: https://github.com/flang-compiler/f18/pull/663