[ASTMatchers] Improve assert message for broken parent map.

Summary:
This assert catches places where the AST (as seen by RecursiveASTVisitor)
becomes disconnected due to incomplete traversal.
Making it print the actual parent-less node is a lot more helpful - it's
possible to work out which part of the tree wasn't traversed.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D56395

llvm-svn: 350612