[PowerPC][XCOFF][MC] Explicitly set containing csect on symbols. [NFC]

Previously we would get the csect a symbol was contained in through its
fragment. This works only if we are writing an object file, and only for
defined symbols. To fix this we set the contating csect explicitly on the
MCSymbolXCOFF object.

Differential Revision: https://reviews.llvm.org/D66032

llvm-svn: 369657