Fix vtbl field addr space

Storing the vtable field of an object should use the same address space as
the this pointer. Currently it is assumed to be addr space 0 but this may not
be true.

This assumption (added in 054cc3b1) caused
issues for the out-of-tree CHERI targets.

Reviewed by: John McCall, Alexander Richardson

Differential Revision: https://reviews.llvm.org/D109841