[NFC][XCOFF][AIX] Rename ControlSections to CsectGroup

The name of ControlSections is not expressive enough to convey what they really are.
CsectGroup can better communicate the concept of grouping csects together since they have similar property.

Reviewer: daltenty

Differential Revision: https://reviews.llvm.org/D69001

llvm-svn: 375021