Warn when #[export_name] is used with generic functions

[bjorn3]

We already warn when using #[no_mangle] with generic functions as it is bound to lead to symbol conflicts.

cc https://rust-lang.zulipchat.com/#narrow/channel/122651-general/topic/Generic.20functions.20with.20.23.5Bunsafe.28export_name.20.3D.20.22.2E.2E.2E.22.29.5D/near/516534422

[mu001999]

@rustbot claim