[flang] Handle USE association in parse tree rewriting

f18 was treating "f() = 1" as a statement function definition
if it could be viewed as being in the specification part and
"f" was a USE-associated function returning a data pointer.
(The non-USE-associated case is fine.)  Fix to allow for "f"
to be USE associated.

Differential Revision: https://reviews.llvm.org/D127430