Improve detecting LLVM 3.5 from Homebrew

The main "llvm" Homebrew package is occasionally getting updated and is
not guaranteed to be version 3.5. Right now it's 3.6, but that depends
on the version of Homebrew that someone has installed.

This lists through all installed Homebrew packages named "llvm*" and
tests each `llvm-config --version` for "3.5*" until it finds one.