[8u20] Request for approval for JDK-8042338 Refactor Types.upperBound to treat wildcards and variables separately

[Dan Smith]

Please approve this backport of langtools fix JDK-8042338 from 9 to 8u20.

Bug report: https://bugs.openjdk.java.net/browse/JDK-8042338
Changeset: http://hg.openjdk.java.net/jdk9/dev/langtools/rev/abbc15936e1b
jdk9 reviewer: vromero

—Dan