RFR: Refresh libclang

[Maurizio Cimadamore]

Following the latest jextract changes, it is now time to refresh the libclang classes used by jextract. As a result, this change will drop `RuntimeHelper` and all constant classes (as the new generation strategy no longer needs them).

I encountered a couple of issues when running the updating script, which prompted me to simplify the script, and to add some more documentation.

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Commit messages:
 - Refresh libclang

Changes: https://git.openjdk.org/jextract/pull/168/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=168&range=00
  Stats: 7505 lines in 36 files changed: 3229 ins; 2808 del; 1468 mod
  Patch: https://git.openjdk.org/jextract/pull/168.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/168/head:pull/168

PR: https://git.openjdk.org/jextract/pull/168