CMAKE_{C,CXX}_COMPILER_TARGET not honoured for clang on Windows

Setting CMAKE_{lang}_COMPILER_TARGET should append --target=${CMAKE_{lang}_COMPILER_TARGET} to the compile line when using either clang (LLVM GNU frontend) or clang-cl (LLVM MSVC frontend) on Windows.

I saw that a recent change -- !5058 (merged) -- addresses this in an unreleased CMake version, but I don't think it's quite complete. That MR:

• only sets CMAKE_${lang}_COMPILE_OPTIONS_TARGET to "--target=" when using the MSVC frontend (clang-cl). It would be useful to set this for the GNU frontend (clang) also.
• has a small typo in CMakeDetermineCXXCompiler.cmake that sets CMAKE_CXX_COPMILER_ID_TEST_FLAGS instead of CMAKE_CXX_COMPILER_ID_TEST_FLAGS, which prevents the --target flag being added during C++ compiler detection.