llvm: Install commandline tools to libexec

Currently llvm package installs only libraries. This is bad thing if people wants to add formulas to homebrew main repo, which depends on the llvm commandline tools like opt, llvm-link, llvm-dis etc. To prevent any conflicts with llvm tools provided by xcode (currently it provides only clang and clang++, not opt nor other useful tools) these hombrew installed binaries are installed to libexec.

Patch was inspired by discussion in https://github.com/mxcl/homebrew/pull/23826