Clean up iwyu code to not be one big if statement.

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.