[8u60] Request for approval 8064803: Javac erroneously uses instantiated signatures when merging abstract most-specific methods

[Maurizio Cimadamore]

Hello,
This is a request for approval to backport a fix from the jdk9 to the 
jdk8u-dev repository.
Changes applies cleanly to jdk8u-dev after path reshuffling.

Bug: https://bugs.openjdk.java.net/browse/JDK-8064803
jdk9 fix: http://hg.openjdk.java.net/jdk9/dev/langtools/rev/6641bd634136

Cheers
Maurizio