CodeGen: Allow annotations on globals in non-zero address space

Summary:
Attribute annotations are recorded in a special global composite variable
that points to annotation strings and the annotated objects.

As a restriction of the LLVM IR type system, those pointers are all
pointers to address space 0, so let's insert an addrspacecast when the
annotated global is in a non-0 address space.

Since this addrspacecast is only reachable from the global annotations
object, this should allow us to represent annotations on all globals
regardless of which addrspacecasts are usually legal for the target.

Reviewers: rjmccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71208