Fix default value for ScalaVersionSetting

The default value was NoScalaVersion before, because tryToSet (where
the default was supposed to be set) is not called at all if the option
is not specified.

The initial value of Xmigration is set to NoScalaVersion (which it was
before, the AnyScalaVersion argument was ignored). AnyScalaVersion
would be wrong, it would give a warning in `Map(1 -> "eis").values`
if the option was not specified. See tests.
SI-6541 valid wildcard existentials for case-module-unapply

Instead of letting the compiler infer the return type of case module
unapply methods, provide them explicitly.

This is enabled only under -Xsource:2.12, because the change is not
source compatible.