[foreign-jextract] RFR: 8267504: jextract should report unsupported language and exit rather than generating partial nonworking code

[Athijegannathan Sundararajan]

Piggybacking to remove unused code. Avoid hardcoding enum values - using generated code instead.

-------------

Commit messages:
 - 8267504: jextract should report unsupported language and exit rather than generating partial nonworking code

Changes: https://git.openjdk.java.net/panama-foreign/pull/539/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=539&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267504
  Stats: 2756 lines in 34 files changed: 1957 ins; 566 del; 233 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/539.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/539/head:pull/539

PR: https://git.openjdk.java.net/panama-foreign/pull/539