* C++: add a function `getDestructors(SILType type, bool isExactType)’: if the type is a final class or `isExactType` is true, then return the one and only destructor of that class.
* swift: add `getDestructor(ofExactType type: Type)` and `getIncompleteCallees`
* swift: remove `getDestructor` from the PassContext. The API of the `calleeAnalysis` can be used instead.
The release-devirtualizer must not run on the same alloc_ref twice.
This is a very rare case, but if it happens it's a very bad thing, because it results in a double-free crash.
The fix is to detect that a dealloc_ref instruction (although it isn't "releasing"), does a memory deallocation.
Found by doing some unrelated experiments.
