Use addUsedDataCons more judiciously in TcDeriv (#17324)

If you derive an instance like this:

deriving <...> instance Foo C

And the data constructors for C aren't in scope, then doDerivInstErrorChecks1 throws an error. Moreover, it will only throw an error if <...> is either stock or newtype. This is because the code that the anyclass or via strategies would generate would not require the use of the data constructors for C.

However, doDerivInstErrorChecks1 has another purpose. If you write this:

import M (C(MkC1, ..., MkCn))

deriving <...> instance Foo C

Then doDerivInstErrorChecks1 will call addUsedDataCons on MkC1 through MkCn to ensure that -Wunused-imports does not complain about them. However, doDerivInstErrorChecks1 was doing this for every deriving strategy, which mean that if <...> were anyclass or via, then the warning about MkC1 through MkCn being unused would be suppressed!

The fix is simple enough: only call addUsedDataCons when the strategy is stock or newtype, just like the other code paths in doDerivInstErrorChecks1.

Fixes #17324 (closed).