Generalize AllocBoxToStack pass to handle apply variations.

[atrick]

Remove all the bespoke utilities for handling apply sites.
Use the ApplySite abstraction instead. As a side effect,
we can now properly analyze begin_apply and try_apply.

[atrick]

@swift-ci test.

[atrick]

@swift-ci test source compatibility.

[atrick]

@eeckstein or @gottesmm would you care to review this?

[gottesmm]

I'll take a look real quick.

[gottesmm]

All of the "refactoring algebra" looks correct to me. LGTM!

[gottesmm]

@atrick I was just thinking. Do we have a test for apply of partial_apply? Would be good to add if it is missing. It may be useful to use a thin_to_thick_function as a test case as well. I am fine with fixing this in a later commit if you want.

[atrick]

@gottesmm thanks. There is an apply of partial_apply test. This PR decreases the need for pass-specific unit tests by removing a bunch of pass specific utilities. It's obviously intractable to test all possible SIL patterns that a pass can handle, but if there's any special-case logic in the pass, that needs an explicit test.

[gottesmm]

SGTM. Was just a random thought.

[atrick]

@swift-ci smoke test and merge.

[atrick]

@swift-ci smoke test and merge.