[libTooling] Change Stencil equality to use `toString()`

Summary:
Removes the `isEqual` method from StencilPartInterface and modifies equality to
use the string representation returned by the `toString` method for comparison.

This means the `run` and `selection` stencils return true by default, and
clients should be cautious in relying on equality operator for comparison of
stencils containing parts generated by these functions.

It also means we no longer need the custom RTTI support (typeId() and
down_cast()), so it has been removed.

Patch by Harshal T. Lehri.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68825

llvm-svn: 374552