CMake: Enable using clang-cl on windows

[enetheru]

As listed here: #1250
clang-cl on windows installed from official llvm website fails because it emulates the msvc interface.

Prior solution assumed that because compiler is clang it uses gnu interface.

CMake offers some helper variables to account for this simulation/emulation of other compilers like:
CMAKE_CXX_COMPILER_FRONTEND_VARIANT

This PR detects the difference and sets the helper variables to match what is in use.
It isn't robust enough to mix and match compiler and linker interfaces though, so using a gnu style linker with an msvc style compiler will fail.

I've tested this on windows and mac, with windows based compilers msvc, msys-gcc msys-clang, llvm, mingw64, without issue.

Fixes #1250

[dsnopek]

Thanks! I haven't tested (Windows isn't my main dev environment), but the code seems OK from a quick skimming, and CI is passing :-)

[enetheru]

I've also fixed the commit message like asked.

[dsnopek]

This one will need a rebase - there's a conflict after having merged a few of your other PRs

[kelteseth]

@enetheru this is exactly what I needed, perfect timing haha 💖. Thank you! I do get the following warning for every file:

[build] [148/3524] Building CXX object xxxx\godot-cpp\CMakeFiles\template_debug.dir\gen\src\classes\gpu_particles_collision_sphere3d.cpp.obj
[build] clang-cl: warning: argument unused during compilation: '/MP31' [-Wunused-command-line-argument]

common_compiler_flags.cmake:78

            # MSVC only
            $<${IS_MSVC}:
                /MP${PROC_N}

31 because my 32 threads Ryzen -1

[enetheru]

Thanks for the catch, looks like clang-cl doesnt care about that flag where MSVC does. I will account for it before I take the PR out of draft. Cheers.

[enetheru]

Rebased ontop of #1648

[dsnopek]

Thanks!