[NCGenerics] fix type lowering

[kavon]

TypeDecl::canBeNoncopyable is never going to be fully accurate. In this instance, we were incorrectly treating some types like S<T: ~Copyable>
as noncopyable, and thus non-trivial, despite them in some cases being
Copyable.

[kavon]

@swift-ci please smoke test

[slavapestov]

TypeDecl::canBeNoncopyable is never going to be fully accurate

Maybe it should be removed to avoid this kind of confusion? Also how is D->canBeNoncopyable() different than D->getDeclaredInterfaceType()->canBeNoncopyable()?

[kavon]

Maybe it should be removed to avoid this kind of confusion?

That's my plan in another PR. Asking the decl is a remnant of the old non-generic world of noncopyables and uses of it need to only be for that legacy code specifically guarded by it.

Also how is D->canBeNoncopyable() different than D->getDeclaredInterfaceType()->canBeNoncopyable()?

There's no TypeBase::canBeNoncopyable because a Type can actually give you an accurate answer, so I called it TypeBase::isNoncopyable.

[kavon]

@swift-ci please smoke test

[kavon]

@swift-ci please smoke test

[drodriguez]

Because it uses an experimental feature, the test needs a REQUIRES: asserts or it will fail in non-assert builds.