RFR: Refresh libclang

[Jorn Vernee]

On Tue, 19 Dec 2023 11:34:40 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> 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.

Marked as reviewed by jvernee (Committer).

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PR Review: https://git.openjdk.org/jextract/pull/168#pullrequestreview-1813131077