build: use clang-cl for Windows CI builds (faster slicing and faster builds) (#15428)

* ci: build the Windows x64 dependencies and slicer with clang-cl

Passes -l -x from the Windows jobs, using the clang-cl and Ninja options
added in #15373. Ninja runs each dependency's build step as a plain command,
so the jobs set up a VC environment for OpenSSL's nmake. arm64 stays on MSVC
for now.

The deps cache key gains the compiler, so windows-x64 becomes
windows-x64-clang and windows-arm64 becomes windows-arm64-msvc.

* deps: select OpenSSL's ARM64 target from DEPS_ARCH

CMAKE_GENERATOR_PLATFORM is only set by -A, which Ninja never receives, so
the Ninja build selected the x64 target on ARM64. DEPS_ARCH is derived from
CMAKE_SYSTEM_PROCESSOR and is already independent of the generator.

* ci: build the Windows ARM64 dependencies and slicer with clang-cl

Three dependencies need handling first. libpng and OpenCV each build an ARM
SIMD path that does not compile with clang-cl, so those paths are off; PNG
already had the same opt-out for Apple ARM. OCCT is built with cl, since
clang-cl cannot emit one of its large generated files and there is no option
to turn that off. All three are gated to Windows ARM64 with clang.

.github/workflows/build_orca.yml

@@ -13,6 +13,10 @@ on:
         arch:
           required: false
           type: string
+        compiler:
+          required: false
+          type: string
+          default: msvc
         macos-combine-only:
           required: false
           type: boolean
@@ -395,7 +399,18 @@ jobs:
         # "tests" builds the unit tests too; the unit_tests_windows_* jobs run them.
         run: |
           $arch = "${{ inputs.arch }}"
-          if ($arch -eq "arm64") { .\build_release_vs.bat slicer arm64 tests } else { .\build_release_vs.bat slicer tests }
+          # -l selects clang-cl and -x Ninja; together they build the slicer with clang.
+          $clang = "${{ inputs.compiler }}" -eq "clang"
+          $flags = if ($clang) { "-l", "-x" } else { @() }
+          if ($clang) {
+            # Build against the same VC toolchain and SDK as the dependencies.
+            $vswhere = "${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe"
+            $vs = & $vswhere -latest -property installationPath
+            $devArch = if ($arch -eq "arm64") { "arm64" } else { "amd64" }
+            Import-Module "$vs\Common7\Tools\Microsoft.VisualStudio.DevShell.dll"
+            Enter-VsDevShell -VsInstallPath $vs -SkipAutomaticLocation -DevCmdArguments "-arch=$devArch"
+          }
+          if ($arch -eq "arm64") { .\build_release_vs.bat slicer arm64 @flags tests } else { .\build_release_vs.bat slicer @flags tests }
         shell: pwsh
 
       - name: Build system preset cache (Windows)