Make IgnoreParens() look through ChooseExprs.

This is the same way GenericSelectionExpr works, and it's generally a
more consistent approach.

A large part of this patch is devoted to caching the value of the condition
of a ChooseExpr; it's needed to avoid threading an ASTContext into
IgnoreParens().

Fixes <rdar://problem/14438917>.

llvm-svn: 186738