When using clang-cl on windows -fvisibility=hidden is not an known
argument. Instead it behaves exactly like MSVC in this case. So let's
make sure we take that path.
In CMake clang-cl sets both CMAKE_C_COMPILER_ID=clang and MSVC get's
defined since clang-cl is basically a MSVC emulator. So guarding like we
do in this patch seems logical.
Reviewed-by: Jakub Zakrzewski
Closes#6194
fd5f35ef0b