[Support] Add stream tie function and use it for errs()

errs() is now tied to outs() so that if something prints to errs(),
outs() will be flushed before the printing occurs. This avoids
interleaving output between the two and is consistent with standard cout
and cerr behaviour.

Reviewed by: labath, JDevlieghere, MaskRay

Differential Revision: https://reviews.llvm.org/D81156