[flang] Selectors whose expressions are pointers returned from functions are valid...

[flang] Selectors whose expressions are pointers returned from functions are valid targets

An ASSOCIATE or SELECT TYPE statement's selector whose "right-hand side" is the result
of a reference to a function that returns a pointer must be usable as a valid target
(but not as a pointer).

Differential Revision: https://reviews.llvm.org/D135211