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

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

> In this case I think just using the hardcoded 0 is less wrong than using the "LangAS::Default". This just happens to work out correctly for the OpenCL 1.x hack on amdgpu  

I don't have a strong preference / that makes sense-ish but I was actually leaning towards switching this to the AllocaAS, which I think is still safe and would do the RightThingTM.

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