[lld-link] Consistently print all /verbose output to stderr

lld-link used to consistently print all /verbose output to stdout, and that was
an intentional decision: https://reviews.llvm.org/rG4bce7bcc88f3

https://reviews.llvm.org/rGe6e206d4b4814 added message() and log(),
and back then `log()` morally was just `if (verbose) message(...)`
and message() wrote to stdout.

So that change moved most /verbose-induced writes to outs() to
log(). Except for the one in printDiscardedMessage(), since
the check for `verbose` for that one is in the caller, in
Writer::createSections():

    if (config->verbose)
      sc->printDiscardedMessage();

Later, https://reviews.llvm.org/D41033 changed log() to write to
stderr. That moved lld-link from writing all its /verbose output
to stdout to writing almost all of its /verbose output to stderr --
except for printDiscardedMessage() output.

This change moves printDiscardedMessage() to call log() as well,
so that all /verbose output once again consistently goes to the same
stream.

Differential Revision: https://reviews.llvm.org/D116667