[clang][Interp] Fix binary comma operators

[tbaeder]

Summary

We left the result of RHS on the stack in case DiscardResult was true.

[shafik]

LGTM

clang/lib/AST/Interp/ByteCodeExprGen.cpp

 bool ByteCodeExprGen<Emitter>::VisitBinaryOperator(const BinaryOperator *BO) {
   // Need short-circuiting for these.
   if (BO->isLogicalOp())
     return this->VisitLogicalBinOp(BO);
 
   const Expr *LHS = BO->getLHS();
   const Expr *RHS = BO->getRHS();
 
-  // Deal with operations which have composite or void types.
-  switch (BO->getOpcode()) {
-  case BO_Comma:
-    if (!discard(LHS))
-      return false;
-    if (!this->visit(RHS))
-      return false;
-    return true;
-  default:
-    break;
-  }
-
   // Typecheck the args.
   std::optional<PrimType> LT = classify(LHS->getType());
   std::optional<PrimType> RT = classify(RHS->getType());
   std::optional<PrimType> T = classify(BO->getType());
   if (!LT || !RT || !T) {
     return this->bail(BO);
   }
 
   auto Discard = [this, T, BO](bool Result) {
     if (!Result)
       return false;
     return DiscardResult ? this->emitPop(*T, BO) : true;
   };
 
+  // Deal with operations which have composite or void types.
+  if (BO->isCommaOp()) {
+    if (!discard(LHS))
+      return false;
+    return Discard(this->visit(RHS));
+  }
+
   // Pointer arithmetic special case.
   if (BO->getOpcode() == BO_Add || BO->getOpcode() == BO_Sub) {
     if (*T == PT_Ptr || (*LT == PT_Ptr && *RT == PT_Ptr))
       return this->VisitPointerArithBinOp(BO);
   }
 
   if (!visit(LHS) || !visit(RHS))
     return false;

clang/test/SemaCXX/constexpr-duffs-device.cpp

 // RUN: %clang_cc1 -std=c++1y -verify %s
+// RUN: %clang_cc1 -std=c++1y -verify -fexperimental-new-constant-interpreter %s
 
 // expected-no-diagnostics
 constexpr void copy(const char *from, unsigned long count, char *to) {
         unsigned long n = (count + 7) / 8;
         switch(count % 8) {
         case 0: do {    *to++ = *from++;
         case 7:         *to++ = *from++;
         case 6:         *to++ = *from++;