[SYCL] set __SYCL_COMPILER_VERSION from CMake variable or git commit timestamp

[blenderfreaky]

This PR changes the way CMake sets __SYCL_COMPILER_VERSION from using the date of compilation to:

• using -DSYCL_COMPILER_VERSION=... if set
• otherwise, running git to get the timestamp of the latest commit, and using that
• falling back to the old behavior if the first two fail

See #19692 for the rationale behind this. TL;DR: Make __SYCL_COMPILER_VERSION reproducible. Closes #19692.

[AlexeySachkov]

Whilst this is definitely better than what we have today, I would deprecate __SYCL_COMPILER_VERSION all together, simply because latest commit timestamp is not a compiler version at all.

My attempt at this is in #16137, but I'd never figured out those pre-commit failures (perhaps using WORKING_DIRECTORY instead of -- $dir is a better approach, the format is also slightly different from what I used: %cd vs %ad) and then it never left my backlog.

[blenderfreaky]

Working directory seems to be what's used in e.g. llvm/cmake/modules/VersionFromVCS.cmake as well.

I think %cd/committer date should be more reliable. %ad/authors date could be too old if for example a PR gets rebase onto master

Wrt. the Pre Commit issues, on your PR the logs are expired. On this one they seem(?) unrelated to the actual changes