[AIX] Make sure to use QualNames for external global objects

Summary: Previously we only handled the case where the csect hadn't been set up yet, so we'd hit an assert later on.

Reviewers: jasonliu, DiggerLin, stevewan

Reviewed By: jasonliu

Subscribers: hubert.reinterpretcast, wuzish, nemanjai, hiraditya, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71032