[llvm-branch-commits] [llvm] release/20.x: [libc++][ci] Update the Windows toolchains to Clang 19 (#129232) (PR #129303)

[Martin Storsjö via llvm-branch-commits]

mstorsjo wrote:

> Does this need to be in 20.1.0 or could it wait until 20.1.1 ?

The libcxx CI gets failures in the clang-cl configs on Windows, which this fixes. So this change isn't urgent in itself, but if the libcxx CI is run on the release branch, it'll have unrelated errors until this is merged.

https://github.com/llvm/llvm-project/pull/129303