Add formula for include-what-you-use

Builds against clang 3.5. See [their homepage][1] for more information.

Was first attempted in #24783. I decided to just use the svn repo since that gets updated more frequently than downloads.

[1]: https://code.google.com/p/include-what-you-use/
There are release tar balls for 3.3 and 3.4 but none against 3.5, which is the `clang` included in the latest OS X and `brew`. The 3.4 release fails to build against clang 3.5's headers (as they mention, the coupling is fairly tight).

In the absence of a release tarball or SCM tag I'm using the latest rev from the 3.5 branch (last committed around September 17).

It's a bit complicated because this _really_ won't build unless you have clang 3.5. I'm completely new to recipes, but thought the homebrew system was mostly around "use the latest thing possible and rely on other packages, not system tools". If this assumption is incorrect (i.e. if we need to support the system clang 3.4 on anything but the latest OS X), tell me.