[ASTContext] Avoid duplicating address space map. NFCI

ASTContext was holding onto a pointer to the Clang->LLVM address space map
which is stored inside TargetInfo. Instead of doing this, we can forward to
TargetInfo instead. This change will allow us to eventually remove
getTargetAddressSpace() from ASTContext and only have this information in
TargetInfo.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D138316