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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196934 91177308-0d34-0410-b5e6-96231b3b80d8