[gold patch] PR gold/13245: don't treat refs from .so as refs from outside IR

[Cary Coutant]

> Please clarify the meaning of the in_real_elf_ field in the definition
> in symtab.h.  Currently the comment says "True if we've seen this symbol
> in a real ELF object."  This patch seems to change that definition
> slightly.
>
> This is OK if you fix the comment.

Done. I also added comments in plugin.cc.

Thanks, committed.

-cary