[Sema][ObjC] Allow conversions between pointers to ObjC pointers and

pointers to structs

clang was just being conservative and trying to prevent users from
messing up the qualifier on the inner pointer type. Lifting this
restriction enables using some of the libc++ templates with ObjC pointer
arguments, which clang currently rejects.

rdar://79018677

Differential Revision: https://reviews.llvm.org/D107021