swift234

Introduce basic support for LLVM vectors as builtins.

This adds builtin types Builtin.VecNxT, where N is a natural number
and T is a builtin type, which map down to the LLVM type <N x T>. 

Update varous builtins to support vector arguments, e.g., binary
operations, comparisons, negation. Add InsertElement and
ExtractElement builtins for vectors.

On top of these builtins, add Vec4f and Vec4b structs to the standard
library, which provide 4xFloat and 4xBool vectors, respectively, with
basic support for arithmetic. These are mostly straw men, to be burned
down at our leisure.

Some issues as yet unresolved:
  - Comparisons of Vec4f'ss are producing bogus Vec4b's, which I
  haven't tracked down yet.
  - We still don't support the shuffle builtin, although it should be
  easy
  - More testing!



Swift SVN r5820