[foreign-jextract] RFR: 8261950: jextract should warn and proceed on unsupported types rather than crashing

[Athijegannathan Sundararajan]

Defensive code to handle unsupported types. piggybacking to fix issues with the test Test8261893.java

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Commit messages:
 - 8261950: jextract should warn and proceed on unsupported types rather than crashing

Changes: https://git.openjdk.java.net/panama-foreign/pull/455/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=455&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261950
  Stats: 72 lines in 3 files changed: 58 ins; 1 del; 13 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/455.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/455/head:pull/455

PR: https://git.openjdk.java.net/panama-foreign/pull/455