Revert r374202"[ObjC generics] Fix not inheriting type bounds in categories/extensions."

This introduced new errors, see below. Reverting until that can be investigated
properly.

  #import <AVFoundation/AVFoundation.h>

  void f(int width, int height) {
    FourCharCode best_fourcc = kCMPixelFormat_422YpCbCr8_yuvs;
    NSDictionary* videoSettingsDictionary = @{
      (id)kCVPixelBufferPixelFormatTypeKey : @(best_fourcc),
    };
  }

  $ clang++ -c /tmp/a.mm

  /tmp/a.mm:6:5: error: cannot initialize a parameter of type
  'KeyType<NSCopying>  _Nonnull const' (aka 'const id') with an rvalue
  of type 'id'
      (id)kCVPixelBufferPixelFormatTypeKey : @(best_fourcc),
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

> 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`.
>
> rdar://problem/54329242
>
> Reviewers: erik.pilkington, ahatanak
>
> Reviewed By: erik.pilkington
>
> Subscribers: jkorous, dexonsmith, ributzka, cfe-commits
>
> Differential Revision: https://reviews.llvm.org/D66696