[llvm] [ThinLTO][NFC] Refactor ThinBackend (PR #110461)

[Teresa Johnson via llvm-commits]

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@@ -197,10 +231,30 @@ class InputFile {
 /// A ThinBackend defines what happens after the thin-link phase during ThinLTO.
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teresajohnson wrote:

Can you update this and have a separate comment for the renamed function type alias and the new struct type?

https://github.com/llvm/llvm-project/pull/110461