RFR JDK-8214700: Compilation fails when extend and implement same method signature with generics

[B. Blaser]

Hi,

The fix for JDK-8207224 being now integrated, please review the
following fix for [1]:

http://cr.openjdk.java.net/~bsrbnd/jdk8214700/webrev.00/

which simply checks that return types are substitutable per JLS §8.4.5
in both direct and inherited implementation.

Thanks (tier1 is OK),
Bernard

[1] https://bugs.openjdk.java.net/browse/JDK-8214700