[clang-cl] missing support for /imsvc switch for system headers
the current clang-cl supports new switch for msvc system headers with suppressed diagnostics.
/imsvc <dir> Add directory to system include search path, as if part of %INCLUDE%
/I <dir> Add directory to include search path
cmake-3.11.0-rc2 still generates /I switch for include_directories( SYSTEM ... ).