[Syntax] Introduce syntax trees

Summary:
A tooling-focused alternative to the AST. This commit focuses on the
memory-management strategy and the structure of the AST.

More to follow later:
  - Operations to mutate the syntax trees and corresponding textual
    replacements.
  - Mapping between clang AST nodes and syntax tree nodes.
  - More node types corresponding to the language constructs.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: llvm-commits, mgorny, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D61637

llvm-svn: 365355