[flang] Fix conformability for intrinsic procedures

There are situations where the arguments of intrinsics must be
conformable, which is defined in section 3.36.  This means they must
have "the same shape, or one being an array and the other being scalar".
But the check we were actually making was that their ranks were the same.

This change fixes that and adds a test for the UNPACK intrinsic, where
the FIELD argument "shall be conformable with MASK".

Differential Revision: https://reviews.llvm.org/D104936