[foreign-memaccess] RFR: JDK-8242046: Add a predicate to determine if MemoryLayout is a padding layout

[Maurizio Cimadamore]

This simple patch adds a predicate to test if a memory layout is a padding layout.

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Commit messages:
 - * Add support for isPadding predicate

Changes: https://git.openjdk.java.net/panama-foreign/pull/86/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/86/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8242046
  Stats: 23 lines in 3 files changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/86.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/86/head:pull/86

PR: https://git.openjdk.java.net/panama-foreign/pull/86