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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163071 91177308-0d34-0410-b5e6-96231b3b80d8