[llvm] [TypeMetadataUtil][CallPromtionUtil]Add utility function getFunctionAtVTableOffset that finds functions through alias. Use it in CallPromotionUtils which didn't promote aliasee previously (PR #80736)

[Teresa Johnson via llvm-commits]

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@@ -77,6 +78,10 @@ void findDevirtualizableCallsForTypeCheckedLoad(
 Constant *getPointerAtOffset(Constant *I, uint64_t Offset, Module &M,
                              Constant *TopLevelGlobal = nullptr);
 
+// Given a vtable, returns the function pointer specified by Offset.
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teresajohnson wrote:

Document the pair contents.

https://github.com/llvm/llvm-project/pull/80736