swift234

Safely implement strict TBAA rules.

This fixes type punning issues with unsafeBitCast.

The optimizer is still too aggressive with UnsafePointer. To fix that,
we first need an explicit API for circumventing type safety
(rdar://23406272).

I should be able to fix the following regressions by migrating the
stdlib away from unsafeBitCast to unsafeReferenceCast (~2 weeks).

Slowdowns:
|.Benchmark.................|..Before.|...After.|.Speedup|
|.ArrayInClass..............|...49.00.|...78.00.|.-37.2%.|
|.Sim2DArray................|..471.00.|..549.00.|.-14.2%.|
|.PrimeNum..................|.1876.00.|.1980.00.|..-5.3%.|

Speedups:
|.Benchmark.................|..Before.|...After.|.Speedup|
|.HeapSort..................|.2962.00.|.2663.00.|..11.2%.|
|.StdlibSort................|.2672.00.|.2537.00.|...5.3%.|