[flang] Added a TODO to include the name of the IMPURE procedure in the error

message for referencing in IMPURE procedure in the scalar-mask-expr of
a DO CONCURRENT.

Also removed a bogus comment from the test.

Both of these changes were in response to review comments.

Original-commit: flang-compiler/f18@4447d59b3982218db131c5ce9a78b8ddb77bfe68
Reviewed-on: https://github.com/flang-compiler/f18/pull/452