Use Visual Studio 2019 in AppVeyor (#3587)

appveyor.yml

@@ -1,7 +1,7 @@
 version: 1.0.{build}
 
 image:
-  - Visual Studio 2017
+  - Visual Studio 2019
   - Ubuntu
 
 platform: x64
@@ -38,7 +38,7 @@ before_build:
 - cmd: call utils\hct\hctstart %HLSL_SRC_DIR% %HLSL_BLD_DIR%
 
 build_script:
-- cmd: call utils\hct\hctbuild -%PLATFORM% -%CONFIGURATION% -vs2017 -spirvtest
+- cmd: call utils\hct\hctbuild -%PLATFORM% -%CONFIGURATION% -spirvtest
 - sh: mkdir build && cd build
 - sh: cmake .. -GNinja $(cat ../utils/cmake-predefined-config-params) -DSPIRV_BUILD_TESTS=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CXX_FLAGS=-Werror
 - sh: ninja