SI-2066 -Xsource:2.10: lenient treatment of variance in <:<, =:=

The soundness hole was exploited in Scalaz. They have fixed their
codebase correctly for Scalac 7.1.x, but have less freedom to
break source compatiblity in 7.0.x.

After this commit, they could choose to compile that branch with
-Xsource:2.10
Merge pull request #3412 from retronym/ticket/2066-2.10-compat

 -Xsource:2.10: lenient treatment of variance in <:<, =:=