[Syntax] Introduce TokenBuffer, start clangToolingSyntax library

Summary:
TokenBuffer stores the list of tokens for a file obtained after
preprocessing. This is a base building block for syntax trees,
see [1] for the full proposal on syntax trees.

This commits also starts a new sub-library of ClangTooling, which
would be the home for the syntax trees and syntax-tree-based refactoring
utilities.

[1]: https://lists.llvm.org/pipermail/cfe-dev/2019-February/061414.html

Reviewers: gribozavr, sammccall

Reviewed By: sammccall

Subscribers: mgrang, riccibruno, Eugene.Zelenko, mgorny, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59887

llvm-svn: 361148