theoretically useful, but not useful in practice.  It adds a bunch of 
complexity, and not much value.  It's best to nuke it.  One big advantage
is that it means the target interfaces will soon lose their SLoc arguments
and target queries can never emit diagnostics anymore (yay).  Removing this
also simplifies some of the core preprocessor which should make it slightly
faster.

Ted, I didn't simplify TripleProcessor, which can now have at most one
triple, and can probably just be removed.  Please poke at it when you have
time.

llvm-svn: 47930