fix Issue 10770 - is(T BASE==enum) with tag enum T - AssertFail:'type' line 428 declaration.c

TypeEnum::toBasetype does not cause fwdref error for named opaque enum type
Merge pull request #2795 from 9rnsr/fix10770

Issue 10770 - is(T BASE==enum) with tag enum T - AssertFail:'type' line 428 declaration.c