Auto merge of #117330 - tmiasko:custom-mir-cleanup-blocks, r=cjgillot

Custom MIR: Support cleanup blocks

Cleanup blocks are declared with `bb (cleanup) = { ... }`.

`Call` and `Drop` terminators take an additional argument describing the unwind action, which is one of the following:

* `UnwindContinue()`
* `UnwindUnreachable()`
* `UnwindTerminate(reason)`, where reason is `ReasonAbi` or `ReasonInCleanup`
* `UnwindCleanup(block)`

Also support unwind resume and unwind terminate terminators:

* `UnwindResume()`
* `UnwindTerminate(reason)`

Author: bors