Object: Don't mark alias unconditionally defined

Summary:
Can't remove EmitAssignment override as llvm/test/Object/X86/nm-bitcodeweak.test
expects this behavior.

Reviewers: pcc, espindola

Subscribers: mehdi_amini, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D44596

llvm-svn: 329651