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

[Matt Arsenault via cfe-commits]

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@@ -49,6 +49,8 @@ class ABIInfo {
   CodeGen::CodeGenTypes &CGT;
   llvm::CallingConv::ID RuntimeCC;
 
+  unsigned getTargetDefaultAS() const;
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arsenm wrote:

I don't really like this as a helper, it's not really expressing any new target concept. It's still just querying what do use for the Default LangAS 

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