[flang] Refine WhyNotModifiable()

The utility routine WhyNotModifiable() needed to become more
aware of the use of pointers in data-refs; the targets of
pointer components are sometimes modifiable even when the
leftmost ("base") symbol of a data-ref is not.

Added a new unit test for WhyNotModifiable() that uses internal
READ statements (mostly), since I/O semantic checking uses
WhyNotModifiable() for all its definability checking.

Differential Revision: https://reviews.llvm.org/D94849