[libTooling][NFC] Refactor implemenation of Transformer Stencils to use standard OOP

Currently, the implementation combines OOP and overloads, using a template to
tie the two together. In practice, this has proven confusing with no
benefits. This patch simplifies the code to use standard OOP design (a
collection of classes deriving from an interface).

Differential Revision: https://reviews.llvm.org/D104317