[ELF] Warn rather than error when duplicate version assignments occur

In lvm2, libdevmapper.so is linked with a version script with duplicate
version assignments:

    DM_1_02_138 { global: ... dm_bitset_parse_list; ... };
    DM_1_02_129 { global: ... dm_bitset_parse_list; ... };

ld.bfd silently accepts this while gold issues a warning. We currently
error, thus inhibit producing the executable. Change the error to
warning to allow this case, and improve the message.

There are some cases where ld.bfd error
`anonymous version tag cannot be combined with other version tags`
but we just warn. It is probably OK for now.

Reviewed By: grimar, ruiu

Differential Revision: https://reviews.llvm.org/D64549

llvm-svn: 365759