[foreign-jextract] RFR: 8259226: jextract should differentiate between clang errors vs other runtime issues

[Athijegannathan Sundararajan]

Introducing ClangException for clang diagnostic errors and returning CLANG_ERROR for diagnostic errors. Also using OUTPUT_ERROR as intended.

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Commit messages:
 - 8259226: jextract should differentiate between clang errors vs other runtime issues

Changes: https://git.openjdk.java.net/panama-foreign/pull/430/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=430&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259226
  Stats: 150 lines in 7 files changed: 138 ins; 2 del; 10 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/430.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/430/head:pull/430

PR: https://git.openjdk.java.net/panama-foreign/pull/430