[flang] Be more persistent in search for non-intrinsic module file

When a particular module name has been brought into a compilation
as an intrinsic module via USE,INTRINSIC, perhaps indirectly via
an enclosing USE statement, f18 will resolve later USE statements
to it unless they are USE,NON_INTRINSIC.  This is not entirely
correct -- a bare USE statement for a module name that happens to
match that of an intrinsic module should still search the search
paths for a non-intrinsic module file of the same name.

Differential Revision: https://reviews.llvm.org/D132163