Add llvm::sys::getHostTriple and remove

Add llvm::sys::getHostTriple and remove
llvm::sys::getOS{Name,Version}.

Right now the implementation just derives from LLVM_HOSTTRIPLE (which
is wrong, but it doesn't look like we have a define for the target
triple). Ideally this routine would actually be able to compute the
triple for targets we care about.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68118 91177308-0d34-0410-b5e6-96231b3b80d8