[Windows] move llvm to toolset (#4487)

images/win/scripts/Tests/LLVM.Tests.ps1

@@ -0,0 +1,7 @@
+Describe "Clang/LLVM" {
+    It "Clang/LLVM installed and version is correct" {
+        $toolsetVersion = (Get-ToolsetContent).llvm.version
+        $clangVersion = clang --version
+        $clangVersion[0] | Should -BeLike "*${toolsetVersion}*"
+    }
+}