[flang] Address name resolution problems

Don't emit a bogus error message about a bad forward reference
when it's an IMPORT of a USE-associated symbol; don't ignore
intrinsic functions when USE-associating the contents of a
module when the intrinsic has been explicitly USE'd; allow
PUBLIC or PRIVATE accessibility attribute to be specified
for an enumerator before the declaration of the enumerator.

Differential Revision: https://reviews.llvm.org/D95175