Fix clang-cl warning flags: use `/W4` instead of `/Wall`

[SquallATF]

This PR fixes the warning flags when building with clang-cl.

Previously, the CMake configuration unconditionally added -Wall -Wextra for Clang. However, according to the clang-cl documentation, /Wall in clang-cl is mapped to -Weverything, which enables all possible warnings, far more than MSVC’s /Wall. This results in a flood of warnings that are not intended.

The original intent was to match MSVC /W4 (≈ -Wall -Wextra in GCC/Clang).