[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:

> There are a still a few places using getTargetAddressSpace(LangAS::Default), which I don't understand.
> 
> Can you also extract my testcase from #115093? I think this should fix that issue (also, this absolutely needs to be fixed in the release branching next week)  

The former was me being daft and missing them. The latter is done (it does fix the issue).

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