SI-8461 -Xsource:2.10 mode for macro signature checks

I can't get the test to fail in partest, so I've resorted to
a manual test case.

    % qbin/scalac -nobootcp -Dscala.usejavacp=false -Xsource:2.10 -classpath $HOME/.m2/repository/org/scala-lang/scala-library/2.10.3/scala-library-2.10.3.jar:$HOME/.m2/repository/org/scala-lang/scala-reflect/2.10.3/scala-reflect-2.10.3.jar test/files/pos/t8461/Impl.scala
    warning: there were 1 deprecation warning(s); re-run with -deprecation for details
    one warning found

    % qbin/scalac -nobootcp -Dscala.usejavacp=false -Xsource:2.11 -classpath $HOME/.m2/repository/org/scala-lang/scala-library/2.10.3/scala-library-2.10.3.jar:$HOME/.m2/repository/org/scala-lang/scala-reflect/2.10.3/scala-reflect-2.10.3.jar test/files/pos/t8461/Impl.scala
    test/files/pos/t8461/Impl.scala:6: error: macro implementations cannot have implicit parameters other than WeakTypeTag evidences
      def reads[A] = macro readsImpl[A]
                           ^
    one error found

Before this change, when using a 2.10 JAR for scala-reflect, the
macro signature validation checks failed. This was due to the fact
that `scala.reflect.macros.Context` was changed in 2.11 to be
a type alias.

To get things working again, I've had to route both
`defintions.{WhiteBoxContextClass, BlackBoxContextClass}` to the
old location. This might mean that we misclassify the boxity under
this mode. All that we can actually handle are blackbox macros, really,
as macro expansion is likely to hit binary incompatibilites very
quickly. We can refine this in subsequent releases.

Review by @huitseeker @xeno-by