llvm: Split llvmFull into separate derivations

Now most packages in the llvm suite are built as separate derivations.
The exceptions are:

* compiler-rt must currently be built with llvm. This increases llvm's
  size by 6 MB
* clang-tools-extra must be built with clang

In addition, the top-level llvm attribute is defaulted to llvm 3.4, and
llvm 3.3 must be accessed by the llvm_33 attribute. This is to make the
out-of-date packages obvious in the hope that eventually all will be
updated to work with 3.4 and 3.3 can be removed. I think we should keep
this policy in the future (latest llvm gets top-level name, the rest are
versioned until they can be removed).

The llvm packages (except libc++, which exception I will try to remove
on the next update) can all be accessed via the llvmPackages attribute,
and there are also aliases for the packages that already existed (llvm,
clang, and dragonegg).

Signed-off-by: Shea Levy <shea@shealevy.com>