Fix build issues on Windows with Clang

[dmatveev]

• By default we passed MSVC flags on Windows unconditionally, extended this behavior to the Clang case;
• Did some tricks to make things work in this configuration;
• Raised the minimum required CMake version based on recent warnings.

Building w/LLVM:

Have LLVM, CMake, and Ninja in your PATH. Then:

cmake ..\ade -DENABLE_ADE_TESTING=ON -DCMAKE_BUILD_TYPE=Release -G Ninja
cmake --build . --target GTest
cmake --build .

Should fix #24

[dmatveev]

Unfortunately ADE's GH Actions don't have Windows.. Opened #37, but meanwhile tested this branch locally with MSVS 2019. Sort-of works

[dmatveev]

@TolyaTalamanov @alexgiving can you please have a look on this too? A write access reviewer approval is required (technically)

[dmatveev]

Thanks!