Recommit r274348 and r274349. The Windows failures should be fixed.

Original commit message:
"Add postorder traversal support to the RecursiveASTVisitor.

This feature needs to be explicitly enabled by overriding shouldTraversePostOrder()
as it has performance drawbacks for the iterative Stmt-traversal.

Patch by Raphael Isemann!

Reviewed by Richard Smith and Benjamin Kramer."

llvm-svn: 274830