[move-function] Add support for properly checking let x: Int without an initial value.

[gottesmm]

We process these as loadable vars. This is really useful since it ensures that
uniqueness is preserved in this case:

  let x: [Enum]
  do {
      x = self.array
  }
  switch _move(x)[userHandle] {
  case .foo:
      assert(self.array._buffer.isUniquelyReferenced()))
  }

I added a test that proves this.

[gottesmm]

@swift-ci smoke test

[gottesmm]

@swift-ci smoke test linux platform

[gottesmm]

@swift-ci smoke test linux platform

[gottesmm]

I looked into the Linux failure! It is actually b/c we run DestroyHoisting. I am just going to manually disable DestroyHoisting on this test since eventually we are going to run it not in the mandatory pipeline and that is what I am testing here.

[gottesmm]

@swift-ci test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 396f510

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 396f510

[gottesmm]

@swift-ci test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 396f510

[gottesmm]

The smoke test signal bot timed out @najacque . I think if I just smoke test it should be fine.

[gottesmm]

@swift-ci smoke test linux platform

[gottesmm]

The actual linux test succeeded though, so I fixed the failed test.

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 396f510

[gottesmm]

MacOS timed out as well

[gottesmm]

@swift-ci smoke test macOS platform