* remove `filterUsers(ofType:)`, because it's a duplication of `users(ofType:)`
* rename `filterUses(ofType:)` -> `filter(usersOfType:)`
* rename `ignoreUses(ofType:)` -> `ignore(usersOfType:)`
* rename `getSingleUser` -> `singleUser`
* implement `singleUse` with `Sequence.singleElement`
* implement `ignoreDebugUses` with `ignore(usersOfType:)`
This is a follow-up of eb1d5f484c.
We cannot do this because we don't know where to insert the compensating release after the propagated `partial_apply`.
A required `strong_retain` may have been moved over the `partial_apply`.
Then we would release the keypath too early.
Fixes a mis-compile
rdar://161321614
(old name: CapturePropagation)
The pass is now rewritten in swift which makes the code smaller and simpler.
Compared to the old pass it has two improvements:
* It can constant propagate whole structs (and not only builtin literals). This is important for propagating "real" Swift constants which have a struct type of e.g. `Int`.
* It constant propagates keypaths even if there are other non-constant closure captures which are not propagated. This is something the old pass didn't do.
rdar://151185177
