Teach the RecursiveASTVisitor to enter parts of the AST previously missed.

Factor its implementation to ease the addition of these custom edges to
traverse. With this patch we get initializer expressions, block bodies, type
source info, and function argument, result, and exception types. There are
probably still some more missed edges.

While we're here, clean up and flesh out a bunch of comments.

Patch by Zhanyong Wan; I've done a cursory review, but further review
appreciated. This is fast becoming one of the most important public APIs to the
AST.

llvm-svn: 104315