Don't discard definitions of common symbols. Not sure if this is the right fix.

Don't discard definitions of common symbols. Not sure if this is the right fix.

Before this change, the program:
  int var;
  int main(void) { return 0; }
when run under 'nm -g' would show 'U var' with the gold plugin and
'B var' with gcc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64616 91177308-0d34-0410-b5e6-96231b3b80d8