[clang] [clang][CodeGen] Add query for a target's flat address space (PR #95728)

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

@jrtc27 @arsenm any additional comments? Are things more palatable in this form? Should this be turned into an RFC? Thanks.

https://github.com/llvm/llvm-project/pull/95728