[flang] Ensure pointer function results are acceptable variables

Fortran permits a reference to a function whose result is a pointer
to be used as a definable variable in any context where a
designator could appear.  This patch wrings out remaining bugs
with such usage and adds more testing.

The utility predicate IsProcedurePointer(expr) had a misleading
name which has been corrected to IsProcedurePointerTarget(expr).

Differential Revision: https://reviews.llvm.org/D98555