Warn when implicitly copying polymorphic data

[msullivan]

As part of #2431, warn when implicitly copying polymorphic data. I'm putting off doing this because I want to get an initial thing landed and don't want to fix all of the warnings for this yet.

[msullivan]

This is actually maybe not the right thing to do... I think what we actually want to do is to shift the burden to call-sites where polymorphic functions are called with not implicitly copyable types.

[msullivan]

I'm going to close this bug as wontfix, and open a new one for what we should actually do.