[foreign-jextract] RFR: 8262851: jextract crashes with "Cannot compute size of a layout which is, or depends on a sequence layout with unspecified size"

[Athijegannathan Sundararajan]

problematic fields are skipped with a warning

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Commit messages:
 - 8262851: jextract crashes with "Cannot compute size of a layout which is, or depends on a sequence layout with unspecified size"

Changes: https://git.openjdk.java.net/panama-foreign/pull/464/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=464&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262851
  Stats: 69 lines in 4 files changed: 63 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/464.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/464/head:pull/464

PR: https://git.openjdk.java.net/panama-foreign/pull/464