Use clang --version to get Apple toolset version

[jkotas]

Fixes #100189

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[filipnavara]

Seems to work both with and without Xcode.

We can switch to the JSON-based ld -version_details for more precise version detection if Apple fixes the new linker (new ld64 is >= 1000).

[VSadov]

Looks good. My only concern is - are we sure the clang version changes in lockstep with xcode on MacOS?
Most likely. I am just not familiar with how they are versioned or whether can be acquired separately and not match.

Technically we just need to find that the clang/xcode is "new enough" to require classic loader switch. Hopefully this whole fallback scheme is temporary.

[VSadov]

LGTM. Thanks!!

[jkotas]

/backport to release/8.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/8406851020