[flang] Disambiguate F(X)=Y case where F is a function returning a pointer

F(X)=Y may be initially parsed as a statement function definition; an
existing pass will detect statement functions that should be rewritten
into assignment statemets with array element references as their
left-hand side variables.  However, F() may also be a reference to a
function that returns a data pointer, and f18 did not handle this
case correctly.

The right fix is to rewrite the parse tree for F(X)=Y into an assignment
to a function reference result.  The cases that are actually assignments
to array elements -- including all of the cases previously handled --
will have their left-hand sides converted to array element references
later by another existing rewriting step.

Differential Revision: https://reviews.llvm.org/D124299