[gold commit] PR 23455: Don't warn by default when discarding version info

[Cary Coutant]

> 2018-08-06  Cary Coutant  <ccoutant@gmail.com>
>
> gold/
>         PR gold/23455
>         * options.h (General_options): Add --warn-drop-version option.
>         * symtab.cc (Symbol_table::set_dynsym_indexes): Check it.

Also cherry-picked to 2.31 branch.

-cary