[foreign-jextract] RFR: 8249949: jextract libclang Cursor children iteration creates a new up call stub every time

[Athijegannathan Sundararajan]

Avoiding repeated upcall stub creation by caching. The array list updated is updated in a synchroinzed method.

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Commit messages:
 - 8249949: jextract libclang Cursor children iteration creates a new up call stub every time

Changes: https://git.openjdk.java.net/panama-foreign/pull/261/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/261/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249949
  Stats: 20 lines in 2 files changed: 12 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/261.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/261/head:pull/261

PR: https://git.openjdk.java.net/panama-foreign/pull/261