The other half of a change made by Enrico for trying to get a correct

triple for a process.  He writes, "Changes to the way setting the
triple works on a target so that if the target has passed a fully
specified triple, and the newly passed triple is not a revamp of
the current one, and the current one is fully specified, then do
not replace the existing triple."

Triple handling got a bit more complicated on mac with the addition
of ios/watchos/tvos and their simulators, and tracking the correct
os versions for them so expressions are compiled with the expected
APIs available to the user.

<rdar://problem/19820698> 

llvm-svn: 252583