Support clang-cl from Windows.Clang.toolchain.cmake

[MarkSchofield]

'Windows.Clang.toolchain.cmake' uses Clang with a GCC front-end, which seems like a great Clang default experience. But as a 'drop-in' replacement for MSVC, or to be able to run clang-tidy on an MSVC codebase, adding clang-cl support would be great. And that's what this PR does. The change:

1. Add support for 'CMAKE_CXX_COMPILER_FRONTEND_VARIANT' to select the clang frontend variant - The crux of the change. 'Windows.Clang.toolchain.cmake' continues to default to a GCC frontend, but by setting CMAKE_C_COMPILER_FRONTEND_VARIANT or CMAKE_CXX_COMPILER_FRONTEND_VARIANT to MSVC will select clang-cl.exe.
2. Convert UTF-16LE files with UTF8 - clang-cl.exe won't parse UTF-16E files, so converting them to UTF8 which Clang-cl and MSVC can load.
3. Add 'clangcl' presets to compile with clang-cl.exe - Adding presets for clang-cl.exe allows the examples to be built with clang-cl.exe.
4. Include 'clangcl' presets in .github/workflows/ci.yaml - Updating the CI definition to run the clangcl presets.