[clang] [clang][CodeGen] `sret` args should always point to the `alloca` AS, so use that (PR #114062)

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

I agree that in theory a target could want to do something else, but that would be an ABI lowering decision. It doesn't naturally come from a source level type. Supporting such a target would require more work, but given the current state of the world just using the alloca address space in this context is the most consistent and simplest decision. 

https://github.com/llvm/llvm-project/pull/114062