JDK 13 RFR of JDK-8224687: Add clarifying overrides of Element.asType to more specific subinterfaces

[Joe Darcy]

Hello,

Please review the webrev and CSR for:

     JDK-8224687: Add clarifying overrides of Element.asType to more 
specific subinterfaces
     webrev: http://cr.openjdk.java.net/~darcy/8224687.2/
     CSR: https://bugs.openjdk.java.net/browse/JDK-8225027

Thanks,

-Joe