Replace all calls with calls to Target.is_synthetic().
In pretty much all cases where we need to check if a target is a codegen
target, what we actually care about is whether that target was synthesized
in order to hold generated code. If didn't use target synthesis as an
implementation detail, we probably wouldn't have needed those is_codegen
checks to begin with.
This is an alternative to https://rbcommons.com/s/twitter/r/4316/ only one
of these will be submitted, hopefully this one, which is simpler.