NFC: Implement AST node skipping in ParentMapContext

Summary:
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

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73388