Fix OpenMP check treating Clang-CL as MSVC

[Error-mdl]

When building with Clang-CL, the OpenMP cmake test sets the flags for MSVC (/openmp:llvm). This is not a valid argument for Clang-CL and the test fails. Only setting the MSVC flags if the compiler ID isn't Clang fixes this.

[HappySeaFox]

Can we simplify the whole if() with just if (CMAKE_C_COMPILER_ID STREQUAL "MSVC")?

[Error-mdl]

Upon further testing, openMP wasn't actually getting enabled when compiling the project. Even though the arguments in OpenMP_C_FLAGS are correct (-Xclang -fopenmp), CMake tries to encapsulate both arguments in a single set of quotes which makes it invalid. Using the alternative syntax for passing normal clang arguments to clang-cl works though (/clang:-fopenmp). This seems to function correctly in my testing:

	if (MSVC)
            if (CMAKE_C_COMPILER_ID STREQUAL "Clang")
                 set(SAIL_OPENMP_FLAGS "/clang:-fopenmp" CACHE INTERNAL "")
            else()
                set(SAIL_OPENMP_FLAGS "/openmp:llvm" CACHE INTERNAL "")
            endif()
        else()
            set(SAIL_OPENMP_FLAGS ${OpenMP_C_FLAGS} CACHE INTERNAL "")
        endif()

[HappySeaFox]

@Error-mdl Thanks for the report! Could you please attach your CMakeCache.txt and compile output when CMake tries to encapsulate both arguments in a single set of quotes here: #205?