[foreign] RFR 8214296: Unify Layout and Function under common supertype

[Maurizio Cimadamore]

Hi,
this is an official RFR for the change already discussed in [1], which 
brings both Layout and Function under a common supertype called Descriptor.

I made few changes from the previously shared patch:

* added copyright on Descriptor.java
* renamed AbstractDescriptor::dup to AbstractDescriptor::withAnnotations
* fixed some minor javadoc errors I encountered when generating/double 
checking public API javadocs
* added test for function annotations parsing

http://cr.openjdk.java.net/~mcimadamore/panama/8214296/

Cheers
Maurizio

[1] - 
http://mail.openjdk.java.net/pipermail/panama-dev/2018-November/003279.html