[clang][bytecode] Fix discarded dynamic casts (llvm#200723)

If they are checked.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -3570,6 +3570,8 @@ bool Compiler<Emitter>::VisitCXXDynamicCastExpr(const CXXDynamicCastExpr *E) {
   if (!this->emitCheckDynamicCast(E))
     return false;
 
+  if (DiscardResult)
+    return this->emitPopPtr(E);
   return true;
 }
 

clang/test/AST/ByteCode/cxx23.cpp

@@ -641,4 +641,8 @@ namespace DynamicCast {
                                                   // all23-note {{dynamic_cast applied to object 'a' whose dynamic type is not constant}} \
                                                   // all20-note {{function parameter 'a' with unknown value cannot be used in a constant expression}}
   }
+
+  struct S {};
+  constexpr S s;
+  constexpr int foo = (dynamic_cast<const S &>(s), 0);
 }