[flang] These are additional changes for issue 458, to perform semantic checks on DO

variable and initial, final, and step expressions.

Here's a summary of the changes since my original pull request:

 - I've taken into account the possibility that the DO variable is declared in
   a MODULE.  This required a call to GetUltimate() on the Symbol for the DO
   variable.
 - The previous change exposed problems in the semantic checking for NULLIFY
   and DEALLOCATE statements, so I've included fixes and tests for those.  I
   also added a test for the ALLOCATE statement, even though it was already
   handling this case.
 - I now handle the case where a procedure name is erroneously used as a DO variable.
 - I now handle the case where a pointer to a procedure is erroneously used as
   a DO variable.
 - I now check that the DO expressions are not null.
 - I added tests for all cases listed above.

Original-commit: flang-compiler/f18@219d856fdb6f36c152153085dbe73eda3442127c
Reviewed-on: https://github.com/flang-compiler/f18/pull/478
Tree-same-pre-rewrite: false