Update appveyor.yml

Author: pdimov

appveyor.yml

@@ -15,18 +15,20 @@ branches:
 environment:
   matrix:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      TOOLSET: msvc-9.0,msvc-10.0,msvc-11.0
-      ADDRMD: 32
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      TOOLSET: msvc-12.0,msvc-14.0
+      TOOLSET: msvc-14.0
+      CXXSTD: 14,latest
       ADDRMD: 32,64
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       TOOLSET: msvc-14.1
-      CXXSTD: 14,17
+      CXXSTD: 14,17,latest
       ADDRMD: 32,64
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: msvc-14.2
-      CXXSTD: 14,17,latest
+      CXXSTD: 14,17,20,latest
+      ADDRMD: 32,64
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      TOOLSET: clang-win
+      CXXSTD: 14,17,20,latest
       ADDRMD: 32,64
 
 install: