AST matcher tutorial (Part I)

This an AST matcher tutorial based on Sam Panzer's document
(https://docs.google.com/a/google.com/document/d/1oTkVLhCdRJUEH1_LDaQdXqe8-aOqT5GLDL9e4MhoFF8/edit).

Checking in now although some parts might be a bit rough so others can
help improving it.

llvm-svn: 169632