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

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-runtime-cuda-clang` running on `as-builder-7` while building `clang` at step 10 "build-flang-runtime-FortranRuntime".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/7/builds/10932

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 10 (build-flang-runtime-FortranRuntime) failure: cmake (failure)
...
      |                           ^~~~~~~~~~~~~~
1 warning generated.
In file included from /home/buildbot/worker/as-builder-7/ramdisk/flang-runtime-cuda-clang/llvm-project/flang/runtime/extrema.cpp:13:
In file included from /home/buildbot/worker/as-builder-7/ramdisk/flang-runtime-cuda-clang/llvm-project/flang/runtime/reduction-templates.h:24:
In file included from /home/buildbot/worker/as-builder-7/ramdisk/flang-runtime-cuda-clang/llvm-project/flang/runtime/numeric-templates.h:22:
In file included from /home/buildbot/worker/as-builder-7/ramdisk/flang-runtime-cuda-clang/llvm-project/flang/runtime/tools.h:17:
/home/buildbot/worker/as-builder-7/ramdisk/flang-runtime-cuda-clang/llvm-project/flang/runtime/../include/flang/Runtime/freestanding-tools.h:115:27: warning: unused function 'MemmoveWrapper' [-Wunused-function]
  115 | static RT_API_ATTRS void *MemmoveWrapper(
      |                           ^~~~~~~~~~~~~~
1 warning generated.
command timed out: 1200 seconds without output running [b'cmake', b'--build', b'.', b'--target', b'FortranRuntime'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1786.866735

```

</details>

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