Create a tool from POC. The POC had some limitation and was too integrated scons.

The new tool should use emitters on the object builders to create the iwyu and fix output files, then set itself as a dependency on the object file, this way when iwyu is enabled, it will automatically happen for current targets. This also will work for ninja.

Create the full-run-iwyu and the git-diff-iwyu targets to facilitate running IWYU.