Github Actions: Add ClangCL on Windows

MS Visual Studio 2019 has builtin support for other compiler toolsets [1].

This commit adds support for compiling using LLVM/Clang 10 using Visual
Studio.

ClangCL pretends to be MSVC so the usual MSVC flags apply, see also [2].
For detecting if ClangCL is used, newer cmake verisons (>= 3.15) have
builtin support using CMAKE_CXX_COMPILER_FRONTEND_VARIANT [3], for older
ones a workaround is available using CMAKE_CXX_SIMULATE_ID [4].

[1]: https://devblogs.microsoft.com/cppblog/clang-llvm-support-in-visual-studio/
[2]: https://clang.llvm.org/docs/UsersManual.html#clang-cl
[3]: https://stackoverflow.com/a/10055571
[4]: https://github.com/microsoft/CMake/commit/4fe34b2d29ca67ea91b2545b223dad8abbcad282

.github/workflows/windows_clang_cl.yml

+name: Windows
+
+on: [push, pull_request]
+
+jobs:
+  clang-cl-10-x64:
+
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: cmake
+      run: cmake -S . -B build -G "Visual Studio 16 2019" -A x64 -T ClangCL -DJSON_BuildTests=On
+    - name: build
+      run: cmake --build build --config Debug --parallel 10
+    - name: test
+      run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure
+
+  clang-cl-10-x86:
+
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: cmake
+      run: cmake -S . -B build -G "Visual Studio 16 2019" -A Win32 -T ClangCL -DJSON_BuildTests=On
+    - name: build
+      run: cmake --build build --config Debug --parallel 10
+    - name: test
+      run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure

README.md

@@ -1254,6 +1254,7 @@ The following compilers are currently used in continuous integration at [Travis]
 | MSVC 19.16.27035.0 (15.9.21+g9802d43bc3 for .NET Framework)     | Windows-10.0.14393 | AppVeyor       |
 | MSVC 19.25.28614.0 (Build Engine version 16.5.0+d4cbfca49 for .NET Framework) | Windows-10.0.17763  | AppVeyor       |
 | MSVC 19.25.28614.0 (Build Engine version 16.5.0+d4cbfca49 for .NET Framework) | Windows-10.0.17763  | GitHub Actions |
+| MSVC 19.25.28614.0 (Build Engine version 16.5.0+d4cbfca49 for .NET Framework) with ClangCL 10.0.0 | Windows-10.0.17763  | GitHub Actions |
 
 ## License