[8u20] Request for approval for JDK-8015101: Covariance of return type implied by upper bounding on type parameter is ignored

[Dan Smith]

Please approve this backport of langtools fix JDK-8015101 from 9 to 8u20.

Bug report: https://bugs.openjdk.java.net/browse/JDK-8015101
Changeset: http://hg.openjdk.java.net/jdk9/dev/langtools/rev/71e241ea30f1
jdk9 reviewer: vromero

—Dan