[clang][bytecode] Fix discarded pointer subtractions (llvm#118477)

We need to pop the value.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -1000,7 +1000,10 @@ bool Compiler<Emitter>::VisitPointerArithBinOp(const BinaryOperator *E) {
     if (!visitAsPointer(RHS, *RT) || !visitAsPointer(LHS, *LT))
       return false;
 
-    return this->emitSubPtr(classifyPrim(E->getType()), E);
+    PrimType IntT = classifyPrim(E->getType());
+    if (!this->emitSubPtr(IntT, E))
+      return false;
+    return DiscardResult ? this->emitPop(IntT, E) : true;
   }
 
   PrimType OffsetType;

clang/test/AST/ByteCode/literals.cpp

@@ -980,6 +980,8 @@ namespace DiscardExprs {
     __uuidof(number); // both-error {{cannot call operator __uuidof on a type with no GUID}}
 
     requires{false;};
+    constexpr int *p = nullptr;
+    p - p;
 
     return 0;
   }