The 'ctest --launch' command must be placed before the IWYU launcher on
the compiler command line. Extend the RunCMake.IncludeWhatYouUse test
to cover this case. The Ninja generator already does it correctly.
Create a <LANG>_INCLUDE_WHAT_YOU_USE target property (initialized by a
CMAKE_<LANG>_INCLUDE_WHAT_YOU_USE variable) to specify an IWYU command
line to be run along with the compiler.