Use the KeyPath implementation's new support for instantiating and dealing with captures to lower the generic context required to dispatch computed accessors with dependent generics.
To get the full benefit of dyld3 on Darwin platforms, pointer relocations need to be pointer-aligned, which unfortunately requires growing some key path data structures a little bit. This does tidy up some code that had to hack around our lack of unaligned load/store operations on UnsafeRawPointer, at least. While we're here, we can also simplify the identification strategy for reabstracted stored properties; we only need the property index to identify, not the absolute offset. rdar://problem/32318829
We need to use the ivar offset variables in this case, since the Swift field offset vector doesn't pick up the adjusted offsets from the ObjC runtime. Fixes SR-5036 | rdar://problem/32488871.
A property imported from Objective-C, or marked in Swift with the `dynamic` keyword, doesn't have a vtable slot, so can't be identified that way. Use the ObjC selector as the unique identifier to ascribe equality to such components. Fixes rdar://problem/31768669. (While we're here, throw some more execution tests and a changelog note in.)
I had optimistically written the code here optimistically hoping #7837 would land in time for me to merge, but that didn't happen, so adjust some things to match the current 12-byte object header size on 32-bit, and introduce some ABI constants for the expected 32- and 64-bit object header sizes we can assert against so that we have some robustness when it eventually changes again. Implements rdar://problem/31768303.
A computed component needs:
- A stable identifier for equality checking. This can be either a static function reference for concrete property implementations, or a reference into a vtable or witness table for a dynamically dispatched property. This should correspond to a public ABI interface for public things.
- Getter and setter (TODO: and materializeForSet) references. These might be thunks to reach the necessary abstraction level for the keypath implementation, so they can't be used as stable identifiers directly.
- Separate out a uniquable KeyPathPattern that describes the context-free shape of the key path, with generic parameters and (eventually) subscript index slots factored out.
- Add component kinds for gettable and settable properties.
