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