[flang] Fix check for assumed-size arguments to SHAPE() & al.

The predicate that is used to detect an invalid assumed-size argument
to the intrinsic functions SHAPE, SIZE, & LBOUND gives false results
for arguments whose shapes are not calculatable at compilation time.
Replace with an explicit test for an assumed-size array dummy argument
symbol.

Differential Revision: https://reviews.llvm.org/D125342