[SymbolGraph] Make symbol-graph output deterministic

SymbolGraph generation iterating over llvm::DenseSet, which makes the
symbols and relationship fields appear in non-deterministic ordering.
Switch to use llvm::SetVector to preserve the insertion order from
SourceEntityWalker to make order deterministic.

Resolves: https://github.com/swiftlang/swift/issues/59602
(cherry picked from commit dfa4a27ec8)

lib/SymbolGraphGen/SymbolGraph.cpp

@@ -650,21 +650,6 @@ void SymbolGraph::serialize(llvm::json::OStream &OS) {
       }
     });
 
-#ifndef NDEBUG
-    // FIXME (solver-based-verification-sorting): In assert builds sort the
-    // edges so we get consistent symbol graph output. This allows us to compare
-    // the string representation of the symbolgraph between the solver-based
-    // and AST-based result.
-    // This can be removed once the AST-based cursor info has been removed.
-    SmallVector<Edge> Edges(this->Edges.begin(), this->Edges.end());
-    std::sort(Edges.begin(), Edges.end(), [](const Edge &LHS, const Edge &RHS) {
-      SmallString<256> LHSTargetUSR, RHSTargetUSR;
-      LHS.Target.getUSR(LHSTargetUSR);
-      RHS.Target.getUSR(RHSTargetUSR);
-      return LHSTargetUSR < RHSTargetUSR;
-    });
-#endif
-
     OS.attributeArray("relationships", [&](){
       for (const auto &Relationship : Edges) {
         Relationship.serialize(OS);