We need to use ObjC dispatch to get to the witness of an ObjC protocol requirement. The wrong answer here was causing us to do the wrong thing when producing identifiers for key paths that refer to ObjC protocol requirements.
This is a bit more robust and user-friendly than hoping more brittle recovery in SILGen or IRGen for unsupported components kicks in. rdar://problem/32200714
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.)
This introduces a few unfortunate things because the syntax is awkward.
In particular, the period and following token in \.[a], \.? and \.! are
token sequences that don't appear anywhere else in Swift, and so need
special handling. This is somewhat compounded by \foo.bar.baz possibly
being \(foo).bar.baz or \(foo.bar).baz (parens around the type), and,
furthermore, needing to distinguish \Foo?.bar from \Foo.?bar.
rdar://problem/31724243
