Port RefChecks fixes from scalac related to linearization

[smarter]

See scala/scala#7439. For some reason, the test in the original issue (scala/bug#11136) does not crash dotty anymore (see #5094), but we haven't ported any of the changes in scala/scala#7439 yet, so we're probably still accepting some broken things.

[smarter]

To be precise, the pos testcase added in #7549 should not compile (as in Scala 2) as it means that a final def is overridden by a non-final def, see also discussion in #11741