[foreign-jextract] RFR: 8244728: jextract generates separate nested class for struct and union typedefs

[Athijegannathan Sundararajan]

* adding a subclass for struct/union typedefs rather than separate classes
* piggybacking fix for MemoryLayout::offset->MemoryLayout::bitOffset change

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

Commit messages:
 - 8244728: jextract generates separate nested class for struct and union typedefs

Changes: https://git.openjdk.java.net/panama-foreign/pull/159/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/159/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244728
  Stats: 81 lines in 4 files changed: 47 ins; 20 del; 14 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/159.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/159/head:pull/159

PR: https://git.openjdk.java.net/panama-foreign/pull/159