[foreign-jextract] RFR: 8252756: jextract does not handle VLAs/Incomplete Arrays/Flexible arrays

[Jorn Vernee]

Hi,

This PR removes the variable length array workarounds that were needed pre-LLVM 9, and adds a test that verifies that
the correct layout is generated for a struct containing a VLA.

Thanks,
Jorn

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Commit messages:
 - Remove VLA workarounds

Changes: https://git.openjdk.java.net/panama-foreign/pull/305/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=305&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252756
  Stats: 106 lines in 5 files changed: 76 ins; 27 del; 3 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/305.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/305/head:pull/305

PR: https://git.openjdk.java.net/panama-foreign/pull/305