[ELF] Add --allow-multiple-definition option.

If --allow-multiple-definition option is given, LLD does not treat duplicate
symbol error as a fatal error. GNU LD supports this option.

Differential Revision: http://llvm-reviews.chandlerc.com/D3211

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205015 91177308-0d34-0410-b5e6-96231b3b80d8