[flang] Changes in response to Peter's comments on a previous pull request.

Specifically, that the conversion of warnings to errors will happen in a
single place in the compiler once we implement warnings as distinct from
errors.  This change made on of the tests invalid.

Also, there's no need to check whether the extension to allow
REAL DO controls is enabled.

Original-commit: flang-compiler/f18@2fae9edd77b930cef8ececb16d719f16477ef3b0
Reviewed-on: https://github.com/flang-compiler/f18/pull/478
Tree-same-pre-rewrite: false