Begin fleshing out more convenience predicates in llvm::Triple and

Begin fleshing out more convenience predicates in llvm::Triple and
convert at least one client over to use them. Subsequent patches both to
LLVM and Clang will try to convert more people over to a common set of
predicates.

This round of predicates is focused on OS-categorization predicates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149815 91177308-0d34-0410-b5e6-96231b3b80d8