[PATCH] Windows: Don't warn when making large unsigned enum values signed

[Reid Kleckner]

  Actually, I take it back.  This is a valuable warning.  When building with MSVC for win64, PointerBitMask is only an int.  We only execute correctly because it gets sign extended back to the full bit mask.  This should never break in practice because there are very few pointers with 32 free bits of alignment, but it's pretty scary.

  I'm going to go fix LLVM.

http://llvm-reviews.chandlerc.com/D1736