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.
Fixes PR39881
Does this pass the tests if you use FullExpr here instead? I don't think we should be treating ConstantExpr and other FullExprs differently.