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

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

> /cherry-pick [b28766e](https://github.com/llvm/llvm-project/commit/b28766eb3f20354d1d7a34ea83b9d915c3715032)

It doesn't cherry-pick cleanly, there are conflicts in the test files

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