[BOLT][NFC] Track fragment relationships using EquivalenceClasses

Three-way splitting can create references between split fragments (warm
to cold or vice versa) that are not handled by
`isChildOf/isParentOf/isChildOrParentOf`. Generalize fragment
relationships to allow checking if two functions belong to one group,
potentially in presence of ICF which can join multiple groups.

Test Plan: NFC for existing tests

Reviewers: maksfb, ayermolo, rafaelauler, dcci

Reviewed By: rafaelauler

Pull Request: https://github.com/llvm/llvm-project/pull/99979

bolt/lib/Core/Exceptions.cpp

@@ -207,7 +207,7 @@ Error BinaryFunction::parseLSDA(ArrayRef<uint8_t> LSDASectionData,
                "BOLT-ERROR: cannot find landing pad fragment");
         BC.addInterproceduralReference(this, Fragment->getAddress());
         BC.processInterproceduralReferences();
-        assert(isParentOrChildOf(*Fragment) &&
+        assert(BC.areRelatedFragments(this, Fragment) &&
                "BOLT-ERROR: cannot have landing pads in different functions");
         setHasIndirectTargetToSplitFragment(true);
         BC.addFragmentsToSkip(this);