[llvm] MC: Merge MCFragment.h into MCSection.h (PR #146315)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Apologies that I am planning to move MCFragment declaration before MCSection.

This is to restore `MCFragment DummyFragment;` within `MCSection` (reverting part of eedc72b45e953bd21cb5c772b8fd24b414894042). This is required to remove `allocInitialFragment` from `MCContext` (MCContext should not be concerned with fragments and resolve some switchSection/changeSection complexity.



https://github.com/llvm/llvm-project/pull/146315