JDK 13 RFR of JDK-8222369: ExecutableElement.getReceiverType returns null instead of NOTYPE

[Joe Darcy]

Hello,

Please review the fix for

     JDK-8222369: ExecutableElement.getReceiverType returns null instead 
of NOTYPE
     http://cr.openjdk.java.net/~darcy/8222369.0/

In the javac implementation, the getReceiverType method is pulled up to 
the root of the hierarchy and is defined to return null.

The hashing in tools/javac/ElementStructureTest will also need to be 
adjusted when this fix goes in since that test does factor the result of 
getReceiver type into its hash.

Thanks,

-Joe