MC: Merge MCFragment.h into MCSection.h

... due to their close relationship. MCSection's inline functions (e.g.
iterator) access MCFragment, and we want MCFragment's inline functions
to access MCSection similarly (#146307).

Pull Request: #146315

Author: MaskRay

llvm/include/llvm/MC/MCFragment.h

@@ -12,7 +12,6 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/MC/MCFixup.h"
-#include "llvm/MC/MCFragment.h"
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCStreamer.h"