This is a bit easier than the fully general case where both the external descriptor and local pattern have captured arguments (because of generics or subscript indices) since we don't have to combine the two argument files in one component.
Client code can make a best effort at emitting a key path referencing a property with its publicly exposed API, which in the common case will match what the defining module would produce as the canonical key path component representation of the declaration. We can reduce the code size impact of these descriptors by not emitting them when there's no hidden or possibly-resiliently-changed-in-the-past information about a storage declaration, having the property descriptor symbol reference a sentinel value telling client key paths to use their definition of the key path component.
The key path pattern needs to include a reference to the external descriptor, along with hooks for lowering its type arguments and indices, if any. The runtime will need to instantiate and interpolate the external component when the key path object is instantiated.
While we're here, let's also reserve some more component header bytes for future expansion, since this is an ABI we're going to be living with for a while.
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.)
