[flang] Catch impure specifics called in DO CONCURRENT

Rework the code used to check for calls to impure procedures in DO CONCURRENT
constructs.  The current code wasn't checking the representation of the
procedure references in the strongly typed expressions, so it was missing
calls to impure subprograms made via generic interfaces.  While here,
improve error messages, and fix some minor issues exposed by testing the
improved checks.

Differential Revision: https://reviews.llvm.org/D155489