Add a distinct hook for `swift_willThrowTypedImpl()`.
This PR adds a hook for `swift_willThrowTypedImpl()` that is distinct from
`swift_willThrow()`. The current implementation of `swift_willThrowTypedImpl()` creates a
temporary existential box for the thrown error and calls the `_swift_willThrow` hook that
is set by XCTest and swift-testing. Unfortunately, this temporary existential box isn't
very useful because its address is not expected to be stable.
Instead, expose a separate hook, `_swift_willThrowTypedImpl`, whose signature matches
that of the new ABI function. This hook can then be used by XCTest and swift-testing to
distinguish between typed throws and untyped throws and avoids creating a temporary
existential box when it won't be useful.
As implemented, if `_swift_willThrowTypedImpl` is not set but `_swift_willThrow` is, the
`swift_willThrowTypedImpl()` will fall back to calling `_swift_willThrow` with a temporary
existential box. We might decide this is unnecessary, but I figured it would allow us more
time to stage adoption of the new hook in the testing libraries.
Resolves rdar://122824443.
