[foreign-jextract] RFR: 8258223: jextract throws exception when unsupport type is used in anonymous struct

[Athijegannathan Sundararajan]

actual unsupported layout is returned from isUnsupported.

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Commit messages:
 - 8258223: jextract throws exception when unsupport type is used in anonymous struct

Changes: https://git.openjdk.java.net/panama-foreign/pull/416/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=416&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258223
  Stats: 93 lines in 4 files changed: 65 ins; 3 del; 25 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/416.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/416/head:pull/416

PR: https://git.openjdk.java.net/panama-foreign/pull/416