[Support] Don't tie errs() to outs() by default

This reverts part of D81156.

Accessing errs() concurrently was safe before and racy after D81156.
(`errs() << 'a'` is always racy)

Accessing outs() and errs() concurrently was safe before and racy after D81156.

Don't tie errs() to outs() by default to fix the fallout.
llvm-dwarfdump is single-threaded and opting in the tie behavior is safe.