Sandboxing llvm33 and other improvements.

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