[libcxx] [ci] Update Clang for Windows jobs to 18.1.x (#95228)

Pick the latest version available in Chocolatey (18.1.6) and llvm-mingw
(20240606, which includes LLVM 18.1.7).

Also add the flag "--allow-downgrade" when installing a specific version
of LLVM. If the preinstalled version is higher than the requested one,
Chocolatey would otherwise error out when requesting installing a lower
version. This will avoid errors in the future, if the runner image comes
preinstalled with a newer version of LLVM.

(This currently seems to happen with a recent version of the GitHub
Actions runner image, version 20240610.1.0 has LLVM 18.1.6 already
preinstalled, and will error out when trying to install the 17.0.6
version that we previously requested.)

Author: mstorsjo

.github/workflows/libcxx-build-and-test.yaml

@@ -214,11 +214,11 @@ jobs:
       - name: Install a current LLVM
         if: ${{ matrix.mingw != true }}
         run: |
-          choco install -y llvm --version=17.0.6
+          choco install -y llvm --version=18.1.6 --allow-downgrade
       - name: Install llvm-mingw
         if: ${{ matrix.mingw == true }}
         run: |
-          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20231128/llvm-mingw-20231128-ucrt-x86_64.zip
+          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20240606/llvm-mingw-20240606-ucrt-x86_64.zip
           powershell Expand-Archive llvm-mingw*.zip -DestinationPath .
           del llvm-mingw*.zip
           mv llvm-mingw* c:\llvm-mingw