Implement the TreeStructure interface through the TextNodeDumper

Summary:
This way, when the generic ASTTraverser is extracted from ASTDumper,
there can't be any problem related to ordering of class members, a
concern that was raised in https://reviews.llvm.org/D55337.

This will also preserve the property that the generic traverser does not
enforce any coupling between the NodeDumper and the TreeStructure.

 https://godbolt.org/z/PEtT1_

Reviewers: aaron.ballman, erichkeane

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D56407

llvm-svn: 350665