Do not allocate space for common symbols with -r

Currently ld.lld -r allocates space for common symbols, whereas ld.bfd
-r doesn't.  As a result the OpenBSD makefile bits for creating libraries
fail as they use ld -X -r to strip local symbols, which results in
duplicate symbol errors because space for the common symbols has been
allocated.

The diff also implements the --define-commons option such that allocation
of commons can be forced even if -r is used.

Patch by Mark Kettenis.

llvm-svn: 292878