From r229162:
  Visual Studio's SAL extension uses a macro named __deallocate. This
  macro is used pervasively
Using -Werror when building for Windows can force the use of -Wno-#warnings
specifically because of this __deallocate #warning. Instead of forcing
builds to disable all #warnings, this option allows libc++ to be built
without this particular warning, while leaving other #warnings enabled.

Patch by Dave Lee!

llvm-svn: 275172