Ignore ConstantExpr in IgnoreParens
This moves it up from IgnoreParenImpCasts to IgnoreParens, so that more
helpers ignore it. For most clients, this ensures that these helpers
behave the same with and without C++17 enabled, which is what appears to
introduce these new expression nodes.
Reviewers: void, rsmith
Differential Revision: https://reviews.llvm.org/D55853