This updates llvm and clang to 3.6.0, while keeping 3.5.0 available as
well.  It creates a new procedure 'clang-from-llvm' that does the
obvious thing.  I want to keep 3.5.0 around for purposes of testing
Guile with various versions of clang.

I've verified that the 3.5.0 packages generate the same derivations as
before, but have not yet finished building 3.6.0 to make sure it works.

      Mark