RFR: JDK-8186688 javax.lang.model.util.Elements.hides does not work correctly with interfaces

[Jonathan Gibbons]

Please review a simple fix to Symbol, to provide the ability to see if 
an element is accessible in a subtype, as distinct from being 
inherited.  This is needed by JavacElements.hides.

The test was provided by Kumar, in the original bug report.

JBS: https://bugs.openjdk.java.net/browse/JDK-8186688
Webrev: http://cr.openjdk.java.net/~jjg/8186688/webrev.00/

-- Jon