[flang] Fix a crash when an array constructor contains an unlimited polymorphic value

Summary:
C7113 States that "An ac-value shall not be unlimited polymorphic."  We failed
to detect this situation which resulted in a crash when trying to get the
underlying derived type specification of the unlimited polymorphic value.

I added code to avoid the crash, code to emit an error message, and a test.

Reviewers: klausler, tskeith, DavidTruby

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83793