Sandboxing llvm formulae and other improvements.

* Put everything in #{prefix}/lib/llvm-#{version}, exception python
  bindings and utility tools.
  - Update path in caveats.
* Suffix binaries with version and symlink to #{bin} to avoid
  conflicts.
  - Not marked as keg_only any more.
  - Also version python bindings.
* llvm33: Add option to build libc++ and ship to location as per
  upstream Makefile.
  - Also update caveat about how to use the included libc++.
* llvm32, llvm33: Add polly support.
  - Build depends on gmp4, isl011, cloog018.
* Drop head links. Development release will be introduced in dedicated
  formula.
* Other formula clean-ups.
* Also make caveats more consistent.