[clang] Avoid duplicating ProgramAddressSpace in TargetInfo. NFCI

This value was added to clang/Basic in D111566, but is only used during
codegen, where we can use the LLVM IR DataLayout instead. I noticed this
because the downstream CHERI targets would have to also set this value
for AArch64/RISC-V/MIPS. Instead of duplicating more information between
LLVM IR and Clang, this patch moves getTargetAddressSpace(QualType T) to
CodeGenTypes, where we can consult the DataLayout.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D138296