Introduce a recursive AST visitor that makes it trivial to recursively

Introduce a recursive AST visitor that makes it trivial to recursively
walk an entire AST, including all of the types, declarations,
statements, and expressions, and allowing one to easily override the
behavior of the walk at any particular node kind.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103308 91177308-0d34-0410-b5e6-96231b3b80d8