LLD: Don't use the stderrOS stream in link before it's reassigned.

Remove the lld::enableColors function, as it just obscures which
stream it's affecting, and replace with explicit calls to the stream's
enable_colors.

Also, assign the stderrOS and stdoutOS globals first in link function,
just to ensure nothing might use them.

(Either change individually fixes the issue of using the old
stream, but both together seems best.)

Follow-up to b11386f9.

Differential Revision: https://reviews.llvm.org/D70492