[foreign-jextract] RFR: JDK-8242045: adjust foreign-jextract code for recent Foreign interface changes

Fri Apr 3 11:39:14 UTC 2020

This patch fixes several issues caused by JDK-8242061, JDK-8242011 and JDK-8241998.

The changes should all be straightforward. When fixing the code generation for VarHandles with MemoryAddress carrier I
also realized that we were translating them as `long` VarHandles. We probably don't have tests for these
getters/setters, and we should add some.

-------------

Commit messages:
 - Fix several issues caused by JDK-8242061, JDK-8242011 and JDK-8241998

Changes: https://git.openjdk.java.net/panama-foreign/pull/95/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/95/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8242045
  Stats: 331 lines in 9 files changed: 14 ins; 42 del; 275 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/95.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/95/head:pull/95

PR: https://git.openjdk.java.net/panama-foreign/pull/95