The /MP flag only applies to cl, not cl frontends to other compilers (e.g. clang-cl, icx-cl etc), whereas the MSVC cmake variable applies to every compiler with a cl frontend. Therefore, we need to reduce the scope of /MP to only MSVC's cl, so that the other cl frontends can compile without warnings.

Fixes #1869

Reproduction steps to test solution on Windows with the LLVM toolchain installed:

cmake -Bbuild -GNinja -DCMAKE_BUILD_TYPE=Release -DCMAKE_LINKER=lld-link -DCMAKE_C_COMPILER=clang-cl -DCMAKE_CXX_COMPILER=clang-cl -DBENCHMARK_DOWNLOAD_DEPENDENCIES=ON
cmake --build build
ctest --test-dir build