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/
Yeah, I mean use `rubocop` or something to let `brew audit` do this work for you.