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/
Nope. It builds _against_ `clang`, not just _with_. Quoth:

> Include-what-you-use makes heavy use of Clang internals, and will occasionally break when Clang is updated. See the include-what-you-use Makefile for instructions on how to keep them in sync.

Maybe Xcode's clang has the dev headers it needs. I'll check.