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

[thakis]

Summary

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.

[hans]

lgtm, nice digging!

lld/COFF/Chunks.cpp

 bool SectionChunk::isCOMDAT() const {
   return header->Characteristics & IMAGE_SCN_LNK_COMDAT;
 }
 
 void SectionChunk::printDiscardedMessage() const {
   // Removed by dead-stripping. If it's removed by ICF, ICF already
   // printed out the name, so don't repeat that here.
   if (sym && this == repl)
-    message("Discarded " + sym->getName());
+    log("Discarded " + sym->getName());
 }
 
 StringRef SectionChunk::getDebugName() const {
   if (sym)
     return sym->getName();
   return "";
 }