[llvm] [libcxx] [ci] Make the CI find the right version of Clang-cl (PR #161736)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

@mstorsjo sorry for the inconvenience and thanks for the fix!

> The libcxx CI is already on Clang 20.x for the mingw build configurations (and likewise for the Linux test configurations, I would presume), but those don't seem to pull in the Clang intrinsics headers. The fact that intrinsics headers are included here seems to be a specific quirk in the MS UCRT headers; since version 10.0.26100.0 of those headers, `wchar.h` seems to pull in `intrin.h` - otherwise this would have been found much sooner.

I think fe42d34274cac79794637bf2f69f85537dde8b74 may be another reason, we include all intrinsics after it. Does it mean we have covered it?

https://github.com/llvm/llvm-project/pull/161736