swift234

stdlib: use fixed-width Builtin.IntXX types for Swift.Int and UInt

Using the unknown-sized Builtin.Word types complicates producing
compile-time overflow diagnostics.  If we don't know the target Word
size, we don't know if there is an overflow.  But SIL optimizer does not
know the size of Word, this is the point of having the Word type in the
first place.

Also, this opens up more possibilities for optimizations.

rdar://17604532

Swift SVN r24788