[flang][NFC] Make semantics test dosemantics03.f90 warning-correct

This is a preparation for D125804, which makes test_errors.py test
warnings the same way it already tests errors, i.e., assert that the
emitted and expected errors are identical. The following changes are
made to the test:

 - Add the WARNING directive where warnings are expected.
 - Remove -Werror in the RUN line. It does not serve much purpose here:
   with -Werror flang makes compilation fail in the presence of
   warnings, but warnings are still printed as warnings and not as
   errors. And I anyway find it better to test the warnings as warnings
   instead of promoting them and test both warnings and errors as
   errors.
 - Update the header comment describing the test case, mostly in
   response to the removal of -Werror.
 - Remove the reference to 'issue 458', referring to
   https://github.com/flang-compiler/f18/issues/458, from the header.
   I think the relevant reference here is to C1120 of the standard,
   and references to bug trackers from other projects (from before
   upstreaming) can be confusing.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D126176