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

[Matt Arsenault via cfe-commits]

================
@@ -1780,6 +1780,14 @@ class TargetInfo : public TransferrableTargetInfo,
     return 0;
   }
 
+  /// \returns Target specific address space for indirect (e.g. sret) arguments.
+  /// If such an address space exists, it must be convertible to and from the
+  /// alloca address space. If it does not, std::nullopt is returned and the
+  /// alloca address space will be used.
+  virtual std::optional<unsigned> getIndirectArgAddressSpace() const {
----------------
arsenm wrote:

This also shouldn't be optional. There always must be a definitive IR address space.

Also I'm not sure I follow why this is still necessary if you've modified getIndirect to carry the address space. RetAI should have this info now? 

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