[ELF] implement --warn-common/--no-warn-common

-warn-common
Warn when a common symbol is combined with another common symbol
or with a symbol definition.  Unix linkers allow  this  somewhat
sloppy  practice, but linkers on some other operating systems do
not.  This option allows you to  find  potential  problems  from
combining global symbols.

Differential revision: http://reviews.llvm.org/D17998

llvm-svn: 263413