SI-6963 Add version to -Xmigration

We decided to make -Xmigration take an argument. But doing so would
break any build that depends on -Xmigration with no arguments. So
this commit adds -Xmigration-from instead. It's nicely mnemonic.

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.