libclang provisionning script needs some work

Current provisioning scripts uses https://download.qt.io/ as base url for downloading pre-built libclang. That works but has 2 downsides:
1. It is slower as the server is not in the CI network
2. The connection is encrypted therefore it can not be cached by the proxy

Impact of the issue is bigger in context of https://bugreports.qt.io/browse/QTBUG-66015 which increases size of the archives significantly.

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The script should use DownloadURL script instead of direct curl, especially that the relevant archives are already mirrored on ci-files01.

A cheap alternative is to replace https:// with http://, but it is a bit backward.

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To update clang version in CI I had to modify a few (~4), not obvious (sha1 is embeded a bit randomly) files. It seems that everything could be handled from a one file which would define all sha1 and base urls and maybe a version.