RFR: JDK-8219147 javadoc should expose covariant return type overrides

[Jonathan Gibbons]

Please review a small change to the VisibleMemberTable, such that 
javadoc will treat methods with a covariant return type as a non-trivial 
override.  This means that such methods will show up explicitly in the 
main summary table for a type, and not be relegated to the "Methods 
declared in" section of the summary table.

-- Jon
JBS: https://bugs.openjdk.java.net/browse/JDK-8219147
Webrev: 
http://cr.openjdk.java.net/~jjg/8219147-covariant-overrides/webrev.00/