[libTooling] Add Stencil constructor.

Summary:
Adds a constructor that takes a vector with which to initialize the `Parts`
field and a corresponding free function that forwards to the constructor. These
definitions are needed to assist in transitioning away from `Stencil` as a class
to defining it as a type alias.

Reviewers: ilya-biryukov

Subscribers: cfe-commits, gribozavr

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69632