NFC: Implement AST node skipping in ParentMapContext
This allows ASTContext to store only one parent map, rather than storing
an entire parent map for each traversal mode used.
This is therefore a partial revert of commit 0a717d5b (Make it possible
control matcher traversal kind with ASTContext, 2019-12-06).
Reviewers: aaron.ballman, rsmith, rnk
Differential Revision: https://reviews.llvm.org/D73388