[clang] [Clang] Set writable and dead_on_unwind attributes on sret arguments (PR #77116)

[John McCall via cfe-commits]

rjmccall wrote:

If I understand the specification of these attributes correctly, they seem fine.  `dead_on_unwind` is definitely fine — the return value must be treated as uninitialized after a call that throws.  `writable` has a somewhat loose specification that scares me a bit, but as long as the actual analysis is being properly conservative about possible aliases created during the call, I think it should be fine.

https://github.com/llvm/llvm-project/pull/77116