[ObjC] For type substitution in generics use a regular recursive type visitor.

Switch to the inheritance-based visitor from the lambda-based visitor to
allow both preorder and postorder customizations during type
transformation. NFC intended.

Reviewers: ahatanak, erik.pilkington

Reviewed By: erik.pilkington

Subscribers: jkorous, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D57075

llvm-svn: 354180