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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> > @mstorsjo Could you take a look here? I've tried a bunch of stuff, but couldn't really get things to run properly.
> 
> Sure, I can try to set this up. I have tested running libcxx tests on Windows GitHub Actions runners before, and got it working before (for a subset of configs, not using the libcxx `run-buildbot` script) - I'll see what I can do.

I got it working for all of our Windows CI configs. See the two topmost commits at https://github.com/mstorsjo/llvm-project/commits/libcxx-gh-win-runners. The one setting up the actions stuff is quite straightforward. The other one fixes a seriously annoying issue in how CMake picks python, please read the commit message thoroughly.

Also, running the libcxx tests on Github runners used to be quite annoying as they used to be quite limited (2 cores, 8 GB memory only), so earlier, I've had to set `-DLIBCXX_EXTRA_SITE_DEFINES="TEST_IS_EXECUTED_IN_A_SLOW_ENVIRONMENT"` and `-DLIBCXX_TEST_PARAMS="large_tests=False"` to make it work reliably - see https://github.com/mstorsjo/llvm-mingw/blob/cfe2ac2783dfd86a7e9ae81aba2f5db3f1f64133/.github/workflows/test-libcxx.yml#L97-L98. But with the new runners it seems to run well without that (although I haven't run all that many rounds yet, so perhaps it fails sporadically still, who knows?).

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