[llvm] [Arm64EC] Correctly handle sret in entry thunks. (PR #92326)

[via llvm-commits]

llvmbot wrote:

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>> /cherry-pick [b28766e](https://github.com/llvm/llvm-project/commit/b28766eb3f20354d1d7a34ea83b9d915c3715032)
>
>It doesn't cherry-pick cleanly, there are conflicts in the test files

Error: Command failed due to missing milestone.

https://github.com/llvm/llvm-project/pull/92326