[ObjC generics] Fix applying `__kindof` to the type parameter.

Fixes the warning about incompatible pointer types on assigning to a
subclass of type argument an expression of type `__kindof TypeParam`.

We already have a mechanism in `ASTContext::canAssignObjCInterfaces`
that handles `ObjCObjectType` with `__kindof`. But it wasn't triggered
because during type substitution `__kindof TypeParam` was represented as
`AttributedType` with attribute `ObjCKindOf` and equivalent type
`TypeArg`. For assignment type checking we use canonical types so
attributed type was desugared and the attribute was ignored.

The fix is in checking transformed `AttributedType` and pushing
`__kindof` down into `ObjCObjectType` when necessary.

rdar://problem/38514910

Reviewers: ahatanak, erik.pilkington, doug.gregor

Reviewed By: doug.gregor

Subscribers: jkorous, dexonsmith, manmanren, jordan_rose, doug.gregor, cfe-commits

Differential Revision: https://reviews.llvm.org/D57076

llvm-svn: 354189