[llvm] [MC] Move MCFragment::Atom to MCSection::Atoms (PR #95341)

[Fangrui Song via llvm-commits]

================
@@ -111,6 +113,9 @@ class MCSection {
   // subsections.
   SmallVector<std::pair<unsigned, FragList>, 1> Subsections;
 
+  // Mach-O only: the defining non-temporary symbol for each fragment.
+  SmallVector<const MCSymbol *, 0> Atoms;
----------------
MaskRay wrote:

We could, but MCSection.cpp doesn't include MCSectionMachO.h and therefore cannot use `Atoms`.
We could make MCSection.cpp include MCSectionMachO.h, but there is still going to be Mach-O concept in generic code.

On the other hand, the number of MCSection instances is small.

https://github.com/llvm/llvm-project/pull/95341