-Wassign-enum: compare unqualified types

This commit changes -Wassign-enum to compare unqualified types.  One could
think that this does not matter much, because who wants a value of enum type
that is const-qualified?  But this breaks the intended pattern to silence this
warning with an explicit cast:

    static const enum Foo z = (enum Foo) 42;

In this case, source type is 'enum Foo', and destination type is 'const enum
Foo', and if we compare qualified types, they don't match, so we used warn.