[libTooling] Introduce new library of source-code builders.
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.
Subscribers: mgorny, cfe-commits, ilya-biryukov
Differential Revision: https://reviews.llvm.org/D67632