[MacOS] Unlink LLVM from brew (#13838)

images/macos/scripts/build/install-llvm.sh

@@ -9,5 +9,7 @@ source ~/utils/utils.sh
 llvmVersion=$(get_toolset_value '.llvm.version')
 
 brew_smart_install "llvm@${llvmVersion}"
+# After brew update to 5.1.0 we have to manually unlink llvm to avoid conflicts with pre-installed llvm on macOS
+brew unlink "llvm@${llvmVersion}"
 
 invoke_tests "LLVM"

images/macos/scripts/tests/LLVM.Tests.ps1

@@ -9,4 +9,9 @@ Describe "Clang/LLVM" {
         $clangVersion = & "$(brew --prefix llvm@$toolsetVersion)/bin/clang" --version
         $clangVersion[0] | Should -BeLike "*${toolsetVersion}*"
     }
+
+    It "Default clang points to Apple clang" {
+        (Get-CommandResult "which clang").Output.Trim() | Should -Be "/usr/bin/clang"
+        (Get-CommandResult "clang --version").Output | Should -BeLike "*Apple clang*"
+    }
 }