Ensure field-annotations on pointers properly match the AS of the field.

Discovered in SYCL, the field annotations were always cast to an i8*,
which is an invalid bitcast for a pointer type with an address space.
This patch makes sure that we create an intrinsic that takes a pointer
to the correct address-space and properly do our casts.

Differential Revision: https://reviews.llvm.org/D109003