Make Triple's isOSBinFormatXXX functions partition triple-space.

Most users would be surprised if "isCOFF" and "isMachO" were simultaneously
true, unless they'd put the compiler in a box with a gun attached to a photon
detector.

This makes sure precisely one of the three formats is true for any triple and
simplifies some target logic based on that.

llvm-svn: 196934