Forbid @objcImpl on root and generic classes

[beccadax]

@_objcImplementation doesn't support two kinds of Objective-C classes:

• Root classes: SILGen doesn't know how to generate code for an Objective-C class with no superclass. And even if it could, ClangImporter doesn't import some of the primitive methods, like -retain and -release, needed to implement a root class.
• Generic classes: Lightweight generics import awkwardly enough that it's actually quite difficult to write extension methods on them. We're declaring them out of scope to avoid these complications.

Diagnose attempts to use the attribute on classes of these kinds.

Fixes rdar://109130979&116066409.

[beccadax]

@swift-ci please test