Backport of fix Clang detection for Visual Studio 2026 (#20316) (#20364)
This backports #20316 to rc, because it is currently a hard show stopper
for builds. This doesn't mean that a new 2026.1.x release is in order.
It just means that, whenever a future RFC in the 2026.1 is necessary,
the branch is ready to accept changes that actually build.