[libclang] The annotation of tokens operation visits statement nodes code-recursively.

This can blow the stack with extremely deep hierarchies. Switch it to data-recursive.

This is implemented by introducing a post-children visitation callback that the
CursorVisitor is calling after child nodes of a cursor have been visited.
This is used by the annotate-tokens visitor to do extra work at that point.

rdar://11979525.

llvm-svn: 163071