[llvm] [MC] Replace fragment ilist with singly-linked lists (PR #95077)

[Fangrui Song via llvm-commits]

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@@ -831,6 +831,20 @@ void MCAssembler::layout(MCAsmLayout &Layout) {
     MCSection *Sec = Layout.getSectionOrder()[i];
     Sec->setLayoutOrder(i);
 
+    // Chain together fragments from all subsections.
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MaskRay wrote:

Since this is not in a bottleneck, I'll keep simple and avoid the condition.

https://github.com/llvm/llvm-project/pull/95077