When computing the availability of a `SILFunction` for linkage the OS version specified in the `@_backDeploy` attribute should be preferred over the version in the `@available` attribute. This ensures that the corresponding symbol is weakly linked when deploying to older OSes than the back deploy "before" version.
Resolves rdar://99962885
The SIL verifier has identified an issue with the SIL generated for property accessors structured like this:
```
public struct S {
@available(macOS, introduced: 12.0)
@_backDeploy(before: macOS 13.0)
public var x: String {
_read {
yield "x"
}
}
}
```
The emitted SIL is invalid because the value `%9` is used after `end_apply` may have ended the lifetime of the value:
```
bb1:
%8 = function_ref @$s4test1SV1xSSvrTwB : $@yield_once @convention(method) (S) -> @yields @guaranteed String9
(%9, %10) = begin_apply %8(%0) : $@yield_once @convention(method) (S) -> @yields @guaranteed String
end_apply %10
yield %9 : $String, resume bb3, unwind bb2
```
The fix is to move the `end_apply` to the resume and unwind blocks, after the value has been yielded to the caller.
Resolves rdar://96879247
