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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> LangAS::Default is not just determined by target. It also depends on language. For OpenCL 1.2 it is private. 

I would have hoped this would be implemented by default assuming a hidden private addrspace qualifier 

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