[TBDGen] Return a vector of symbols instead of set

A couple of clients are iterating over the result,
so switch to a vector to ensure we don't
accidentally introduce any non-determinism.

lib/FrontendTool/FrontendTool.cpp

@@ -1074,7 +1074,7 @@ static bool writeLdAddCFileIfNeeded(CompilerInstance &Instance) {
     llvm::raw_svector_ostream NameOS(NameBuffer);
     NameOS << "ldAdd_" << Idx;
     OS << "extern const char " << NameOS.str() << " __asm(\"" <<
-      changeToLdAdd(S.getKey()) << "\");\n";
+      changeToLdAdd(S) << "\");\n";
     OS << "const char " << NameOS.str() << " = 0;\n";
     ++ Idx;
   }