SI-6963 Add version to -Xmigration

Adds an optional version parameter to the -Xmigration compiler setting.
Doing -Xmigration without version number behaves as it used to by
dumping every possible migration warning.

This commit adds a ScalaVersion class (plus ancillary stuff), and a
ScalaVersionSetting.
Merge pull request #1998 from JamesIry/2.10.x_6963_2

SI-6963 Add version to -Xmigration