Now that the implementation of all of the Expr::Ignore* is in Expr.cpp we can try to remove some duplication. Do this by separating the logic of the Expr::Ignore* from the iterative loop.
This is NFC, except for one change: IgnoreParenImpCasts now skips, among other things, everything that IgnoreImpCasts skips. This means FullExpr are now skipped by IgnoreParenImpCasts. This was likely an oversight when FullExpr was added to the nodes skipped by IgnoreImpCasts.