MC: Declare MCFragment before MCSection
... so that we can restore `MCFragment DummyFragment;` within
`MCSection` (reverting part of
eedc72b45e953bd21cb5c772b8fd24b414894042).
This is required to remove `allocInitialFragment` from `MCContext`
and postpone initial fragment creation to `changeSection`
to address some design issues.
Pull Request: https://github.com/llvm/llvm-project/pull/150180
