[AIX] Avoid unset csect assert for functions defined after their use in TOC

Summary:
If a function is defined after it appears in a TOC expression, we may
try to access an unset containing csect when returning a symbol for the
expression.

Reviewers: Xiangling_L, DiggerLin, jasonliu, hubert.reinterpretcast

Reviewed By: hubert.reinterpretcast

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71125