Auto merge of #81574 - tmiasko:p, r=oli-obk

Auto merge of #81574 - tmiasko:p, r=oli-obk

Precompute ancestors when checking privacy

Precompute ancestors of the old error node set so that check for private
types and traits in public interfaces can in constant time determine if
the current item has any descendants in the old error set.

This removes disparity in compilation time between public and private type
aliases reported in #50614 (from 30 s to 5 s, in an example making extensive use
of private type aliases).

No functional changes intended.