[ASTMatchers] Improve assert message for broken parent map.

[sammccall]

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.

[aaron.ballman]

LGTM!

lib/ASTMatchers/ASTMatchFinder.cpp

bool matchesAncestorOfRecursively(const ast_type_traits::DynTypedNode &Node,
     const auto &Parents = ActiveASTContext->getParents(Node);
     if (Parents.empty()) {
       // Nodes may have no parents if:
       //  a) the node is the TranslationUnitDecl
       //  b) we have a limited traversal scope that excludes the parent edges
       //  c) there is a bug in the AST, and the node is not reachable
       // Usually the traversal scope is the whole AST, which precludes b.
       // Bugs are common enough that it's worthwhile asserting when we can.
-      assert((Node.get<TranslationUnitDecl>() ||
-              /* Traversal scope is limited if none of the bounds are the TU */
-              llvm::none_of(ActiveASTContext->getTraversalScope(),
-                            [](Decl *D) {
-                              return D->getKind() == Decl::TranslationUnit;
-                            })) &&
-             "Found node that is not in the complete parent map!");
+#ifndef NDEBUG
+      if (!Node.get<TranslationUnitDecl>() &&
+          /* Traversal scope is full AST if any of the bounds are the TU */
+          llvm::any_of(ActiveASTContext->getTraversalScope(), [](Decl *D) {
+          })) {
+        llvm::errs() << "Tried to match orphan node:\n";
+        Node.dump(llvm::errs(), ActiveASTContext->getSourceManager());
+        llvm_unreachable("Parent map should be complete!");
+      }
+#endif
       return false;
     }
     if (Parents.size() == 1) {
       // Only one parent - do recursive memoization.
       const ast_type_traits::DynTypedNode Parent = Parents[0];
       BoundNodesTreeBuilder BuilderCopy = *Builder;
       if (Matcher.matches(Parent, this, &BuilderCopy)) {
         *Builder = std::move(BuilderCopy);