It's kind of unclear what `*-windows-none` actually means, but as far as LLVM is
concerned, it's equivalent to `*-windows-msvc`. For clarity, only test
`*-windows-msvc` and `*-windows-gnu`. #20690 will clean this situation up
eventually.
Also ensure coverage of `link_libc = true` and `link_libc = false` for both.
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