[foreign-jextract] RFR: 8251943: jextract should not generate MemorySegment typed fields for variables, struct fields if layout size info is not available

[Athijegannathan Sundararajan]

problematic variables, struct/union fields are ignored with a warning.

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Commit messages:
 - 8251943: jextract should not generate MemorySegment typed fields for variables, struct fields if layout size info is not available

Changes: https://git.openjdk.java.net/panama-foreign/pull/284/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/284/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251943
  Stats: 38 lines in 2 files changed: 25 ins; 4 del; 9 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/284.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/284/head:pull/284

PR: https://git.openjdk.java.net/panama-foreign/pull/284