Now that the IWYU cleanup has been merged compile all, not just a couple, of
the builtin modules independent of builtin.cpp. That is, no longer `#include
builtin_NAME.cpp` in builtin.cpp. This is more consistent, more in line with
what developers expect, and is likely to reduce mistakes.
Reduces lint errors from 384 to 336 (-13%). Line count from 6307 to 4988 (-21%).
Another step in resolving issue #2902.
Remove the "make iwyu" build target. Move the functionality into the
recently introduced lint.fish script. Fix a lot, but not all, of the
include-what-you-use errors. Specifically, it fixes all of the IWYU errors
on my OS X server but only removes some of them on my Ubuntu 14.04 server.
Fixes#2957
