[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 [fe42d34](https://github.com/llvm/llvm-project/commit/fe42d34274cac79794637bf2f69f85537dde8b74) may be another reason, we include all intrinsics after it.
> 
> I don't think that commit is related here. It's in 21.x only, and I was hitting this on 20.x. And that commit is a no-op if `__SCE__` wasn't defined before.
> 
> > Does it mean we have covered it?
> 
> I don't understand what you mean by this question here.

I mean with fe42d34, these intrinsics should be tested on Windows environment since they are included through `intrin.h`.

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