[CI][Github] Install Clang in Windows container

Downloads clang-for-windows from the LLVM releases website,
decompresses and untars the images, and leave them in
C:\clang\clang-msvc\...  Temporarily downloads the 'xz' utility
to decompress the downloaded clang tarball image.

Author: cmtice

.github/workflows/containers/github-action-ci-windows/Dockerfile

@@ -98,3 +98,37 @@ RUN powershell -Command \
     Add-Type -AssemblyName System.IO.Compression.FileSystem ; \
     [System.IO.Compression.ZipFile]::ExtractToDirectory('actions-runner-win.zip', $PWD) ;\
     rm actions-runner-win.zip
+
+# Download and extract Clang compiler.
+# Create directories, download, extract, and clean up all in one layer
+RUN powershell -Command \
+    # --- Setup directories --- \
+    Write-Host "Creating directories..."; \
+    New-Item -Path "C:\temp-download" -ItemType "Directory" -Force ; \
+    New-Item -Path "C:\xz-utils" -ItemType "Directory" -Force ; \
+    New-Item -Path "C:\clang" -ItemType "Directory" -Force ; \
+    # --- 1. Download and extract xz --- \
+    Set-Location C:\temp-download ; \
+    Write-Host "Downloading xz-utils..."; \
+    Invoke-WebRequest -Uri "http://github.com/tukaani-project/xz/releases/download/v5.8.1/xz-5.8.1-windows.zip" -OutFile "xz.zip"; \
+    Write-Host "Extracting xz-utils..."; \
+    Add-Type -AssemblyName "System.IO.Compression.FileSystem"; \
+    [System.IO.Compression.ZipFile]::ExtractToDirectory('C:\temp-download\xz.zip', 'C:\xz-utils'); \ 
+    # --- 2. Download and decompress Clang --- \
+    Write-Host "Downloading Clang..."; \
+    Invoke-WebRequest -Uri "http://github.com/llvm/llvm-project/releases/download/llvmorg-21.1.2/clang+llvm-21.1.2-x86_64-pc-windows-msvc.tar.xz" -OutFile "clang+llvm-21.1.2-x86_64-pc-windows-msvc.tar.xz" ; \
+    Write-Host "Decompressing clang.tar.xz using C:\xz-utils\bin_x86-64\xz.exe"; \
+    C:\xz-utils\bin_x86-64\xz.exe -d -qq clang+llvm-21.1.2-x86_64-pc-windows-msvc.tar.xz ; \
+    # --- 3. Extract clang --- \
+    Write-Host "Extracting clang.tar to C:\clang ..."; \
+    C:\Windows\System32\tar.exe -xf clang+llvm-21.1.2-x86_64-pc-windows-msvc.tar -C C:\clang ; \
+    # --- 4. Clean up --- \
+    Write-Host "Cleaning up..." ; \
+    Set-Location C:\ ; \
+    Remove-Item C:\temp-download -Recurse -Force; \
+    Remove-Item C:\xz-utils -Recurse -Force; \
+    Write-Host "Download and extraction complete." ;
+
+RUN powershell -Command \
+    Set-Location C:\clang ; \
+    Rename-Item -Path "C:\clang\clang+llvm-21.1.2-x86_64-pc-windows-msvc" -NewName "C:\clang\clang-msvc" ;