[Sema] Remove assert in TreeTransform<Derived>::TransformObjCObjectType.

The assert isn't correct since TypeLoc::ObjCObjectTypeLoc doesn't
indicate whether the type is a dependent type. The function returns
false for a type like "<SomeProtocol>" which is a synonym for
"id<SomeProtocol>".

rdar://problem/23838912

Differential Revision: http://reviews.llvm.org/D17355

llvm-svn: 261829