RFR: 8257184: Upstream 8252504: Add a method to MemoryLayout which returns a offset-computing method handle

[Jorn Vernee]

This upstreams the patch from: https://github.com/openjdk/panama-foreign/pull/396

There were only some minor merge conflicts due to imports and some tests being replaced by java/foreign/TestNulls. All tests still pass, no other changes were needed.

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Commit messages:
 - Add a method to MemoryLayout which returns a offset-computing method handle

Changes: https://git.openjdk.java.net/jdk/pull/1468/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1468&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257184
  Stats: 312 lines in 4 files changed: 255 ins; 7 del; 50 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1468.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1468/head:pull/1468

PR: https://git.openjdk.java.net/jdk/pull/1468