[flang] Catch attempts to copy pointers in allocatables in PURE

In a pure context, a pointer acquired from an INTENT(IN) dummy argument
may not be copied.  Catch the case in which the pointer is a component
of an allocatable component at some depth of nesting.

(This patch adds a new component iterator kind that is a variant of
a potential subobject component iterator; it visits all potential
subobject components, plus pointers, into which it does not descend.)

Differential Revision: https://reviews.llvm.org/D139161