[clang-diff] Add initial implementation

This is the first commit for the "Clang-based C/C++ diff tool" GSoC project.

ASTDiff is a new library that computes a structural AST diff between two ASTs
using the gumtree algorithm. Clang-diff is a new Clang tool that will show
the structural code changes between different ASTs.

Patch by Johannes Altmanninger!

Differential Revision: https://reviews.llvm.org/D34329

llvm-svn: 308731