[ObjC generics] Fix not inheriting type bounds in categories/extensions.
When a category/extension doesn't repeat a type bound, corresponding
type parameter is substituted with id when used as a type argument. As
a result, in the added test case it was causing errors like
type argument 'T' (aka 'id') does not satisfy the bound ('id<NSCopying>') of type parameter 'T'
We are already checking that type parameters should be consistent
everywhere (see checkTypeParamListConsistency) and update
ObjCTypeParamDecl to have correct underlying type. And when we use the
type parameter as a method return type or a method parameter type, it is
substituted to the bounded type. But when we use the type parameter as a
type argument, we check ObjCTypeParamType that ignores the updated
underlying type and remains id.
Fix by desugaring ObjCTypeParamType to the underlying type, the same
way we are doing with TypedefType.
Reviewers: erik.pilkington, ahatanak
Reviewed By: erik.pilkington
Subscribers: jkorous, dexonsmith, ributzka, cfe-commits
Differential Revision: https://reviews.llvm.org/D66696