Create a <LANG>_CLANG_TIDY target property (initialized by a
CMAKE_<LANG>_CLANG_TIDY variable) to specify a clang-tidy command line
to be run along with the compiler.
Add an internal "cmake -E __run_iwyu" mode to wrap the compiler call.
Run a given include-what-you-use command line with the compiler options
and report a warning if it finds anything. Then run the real compiler.
Co-Author: Brad King <brad.king@kitware.com>