Introduce BoundNodes::getMap.

The purpose of this function is to allow clients of the dynamic AST matcher
to enumerate each binding.

Differential Revision: http://llvm-reviews.chandlerc.com/D2095
Introduce DynTypedNode::print, dump and getSourceRange.

These functions can generally be applied to multiple kinds of AST node,
so it makes sense to add them to DynTypedNode.

Differential Revision: http://llvm-reviews.chandlerc.com/D2096