[llvm] [libcxx] [libc++] Use GitHub-provided runners for the windows CI (PR #79326)

[Louis Dionne via llvm-commits]

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@@ -199,3 +199,44 @@ jobs:
             **/CMakeError.log
             **/CMakeOutput.log
             **/crash_diagnostics/*
+  windows:
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ldionne wrote:

When we're ready to merge this, we should add `needs: [ stage1, stage2, stage3 ]` to only trigger the Windows CI if the other stages are happy.

https://github.com/llvm/llvm-project/pull/79326