Add Clang 12 to Ubuntu 20.04 toolset (#3206)

* Add Clang 12 to Ubuntu 20.04 toolset

* Ubuntu: Install Clang-12 directly from LLVM

images/linux/scripts/installers/clang.sh

@@ -12,7 +12,7 @@ function InstallClang {
     local version=$1
 
     echo "Installing clang-$version..."
-    if [[ $version =~ 9 ]] && isUbuntu16; then
+    if [[ $version =~ 9 ]] && isUbuntu16 || [[ $version =~ 12 ]]; then
         ./llvm.sh $version
         apt-get install -y "clang-format-$version"
     else

images/linux/toolsets/toolset-2004.json

@@ -247,7 +247,8 @@
         "versions": [
             "9",
             "10",
-            "11"
+            "11",
+            "12"
         ],
         "default_version": "10"
     },