[clang][Interp] Fix ignoring Integral- and IntegralToBoolean casts

[cor3ntin]

LGTM

clang/lib/AST/Interp/ByteCodeExprGen.cpp

bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
   case CK_UserDefinedConversion:
   case CK_BitCast:
     if (DiscardResult)
       return this->discard(SubExpr);
     return this->visit(SubExpr);
 
   case CK_IntegralToBoolean:
   case CK_IntegralCast: {
+      if (DiscardResult)
+        return this->discard(SubExpr);
     std::optional<PrimType> FromT = classify(SubExpr->getType());
     std::optional<PrimType> ToT = classify(CE->getType());
     if (!FromT || !ToT)
       return false;
 
     if (!this->visit(SubExpr))
       return false;
 

clang/test/AST/Interp/literals.cpp

constexpr int ignoredExprs() {
     (int[]){1,2,3};
     int arr[] = {1,2,3};
     arr[0];
     "a";
     'b';
     sizeof(int);
     alignof(int);
 
+    (short)5;
+    (bool)1;
+
     return 0;
   }
   static_assert(ignoredExprs() == 0, "");
 
   constexpr int oh_my(int x) {
     (int){ x++ };
     return x;
   }