[AST] Factor out the logic of the various Expr::Ignore*

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.

Differential Revision: https://reviews.llvm.org/D57267

Reviewed By: aaron.ballman (with comments from void and rnk)

llvm-svn: 354215