[libTooling] Add `toString` method to the Stencil class

Summary:
`toString` generates a string representation of the stencil.

Patch by Harshal T. Lehri.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68574

llvm-svn: 373916