[NFC][XCOFF][AIX] Refactor get/setContainingCsect

Summary:
For current architect, we always require setContainingCsect to be
called on every MCSymbol got used in XCOFF context.
This is very hard to achieve because symbols gets created everywhere
 and other MCSymbol types(ELF, COFF) do not have similar rules.
It's very easy to miss setting the containing csect, and we would
need to add a lot of XCOFF specialized code around some common code area.

This patch intendeds to do
1. Rely on getFragment().getParent() to get csect from labels.
2. Only use get/setRepresentedCsect (was get/setContainingCsect)
if symbol itself represents a csect.

Reviewers: DiggerLin, hubert.reinterpretcast, daltenty

Differential Revision: https://reviews.llvm.org/D77080