swift234

stdlib: Fix unary - for floats.

+0.0 - ±0.0 == ±0.0, so the correct definition of '-x' is '-0.0 - x'. However, this would be infinitely recursive, so I added an 'fneg' builtin that lowers directly to 'fsub -0.0, %x', and redefined the unary - operators for floats in terms of it.

Swift SVN r4634