Refactor and expand @objcImpl checking
Create a checker for @_objcImplementation member implementations that considers all of a class’s interface and implementation decls at once. This allows us to handle several things better:
• Unimplemented requirements are now diagnosed
• Header members that can match several implementations, or implementations that could match several header members, are now diagnosed
• Tailored diagnostic when the implementation's Swift name matches the header's selector instead of its Swift name
• Recommends inserting `@objc(<selector>)` when a Swift name matches but the implicit ObjC name doesn't
• An `@objc(<selector>)` on one implementation can eliminate its requirement from being considered for other implementations, resolving ambiguities
This does unfortunately regress the diagnostics when a requirement is implemented in the wrong extension. Some sort of whole-module checking would be needed to address this problem.
