Clean up and slightly generalize implementation of composite pointer

type computation, in preparation for P0388R4, which adds another few
cases here.

We now properly handle forming multi-level composite pointer types
involving nested Objective-C pointer types (as is consistent with
including them as part of the notion of 'similar types' on which this
rule is based). We no longer lose non-CVR qualifiers on nested pointer
types.

Author: zygoloid