[libTooling] Introduce new library of source-code builders.

Summary:
Introduces facilities for easily building source-code strings, including
idiomatic use of parentheses and the address-of, dereference and member-access
operators (dot and arrow) and queries about need for parentheses.

Reviewers: gribozavr

Subscribers: mgorny, cfe-commits, ilya-biryukov

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67632

llvm-svn: 372595