Refactor tree printing in AST dumping.

Refactor tree printing in AST dumping.

Instead of manually maintaining a flag indicating whether we're about to print
out the last child of the parent node (to determine whether we print "`" or
"|"), capture a callable to print that child and defer printing it until we
either see a next child or finish the parent.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220930 91177308-0d34-0410-b5e6-96231b3b80d8