Do not construct string from nullptr

[georgthegreat]

Summary

While I am trying to forbid such usages systematically in https://reviews.llvm.org/D79427 / P2166R0 to C++ standard, this PR fixes this (definitelly incorrect) usage in llvm.

[mclow.lists]

LGTM.

[nikic]

(Marking as accepted per above.)

[georgthegreat]

I have no commit access to llvm monorepo, so could, please, someone land this commit?

llvm/utils/TableGen/DFAEmitter.cpp

 //===----------------------------------------------------------------------===//
 
 namespace {
 // FIXME: This entire discriminated union could be removed with c++17:
 //   using Action = std::variant<Record *, unsigned, std::string>;
 struct Action {
   Record *R = nullptr;
   unsigned I = 0;
-  std::string S = nullptr;
+  std::string S;
 
   Action() = default;
   Action(Record *R, unsigned I, std::string S) : R(R), I(I), S(S) {}
 
   void print(raw_ostream &OS) const {
     if (R)
       OS << R->getName();
     else if (!S.empty())