[flang] Validate SIZE(x,DIM=n) dimension for assumed-size array x

[flang] Validate SIZE(x,DIM=n) dimension for assumed-size array x

Catch invalid attempts to extract the unknowable extent of the last
dimension of an assumed-size array dummy argument, and clean up
problems with assumed-rank arguments in similar circumstances
exposed by testing the fix.

Differential Revision: https://reviews.llvm.org/D109918