[Profiler] Clean up walking code a little
Use the new explicit ASTWalker actions to factor
out the skipping logic, ensuring to apply it in
post-visitation too, which fixes a PCMacro crash
(though I don't believe this is a configuration
that ever actually comes up). Also tighten up
invariants such that an `IfExpr` must appear
within an existing region.
coverage_pc_macro.swift