This commit changes the internal -E__run_iwyu to be -E__run_co_compile. This
is used for co-compile commands. These are tools that want to mirror the
compiler. For each compiler invocation the tool will be invoked first. This
started as a way to implement include what you use (iwyu), but has expanded
to include cpplint, cppcheck and others. Likely there will be more in the
future as well. This commit implements each one in its own function and
provides a way to add additional ones in the future with less work.
Create a `<LANG>_CPPCHECK` target property (initialized by a
`CMAKE_<LANG>_CPPCHECK` variable) to specify a `cppcheck` command line
to be run along with the compiler.
