[clang][bytecode] Don't outright reject dyamic casts

Just delegate to the subexpr instead.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -773,6 +773,11 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   case CK_ToVoid:
     return discard(SubExpr);
 
+  case CK_Dynamic:
+    // This initially goes through VisitCXXDynamicCastExpr, where we emit
+    // a diagnostic if appropriate.
+    return this->delegate(SubExpr);
+
   default:
     return this->emitInvalid(CE);
   }

clang/test/AST/ByteCode/cxx20.cpp

@@ -1201,3 +1201,13 @@ namespace NonPureVirtualCall {
 
   int main() { check(); }
 }
+
+namespace DyamicCast {
+  struct X {
+    virtual constexpr ~X() {}
+  };
+  struct Y : X {};
+  constexpr Y y;
+  constexpr const X *p = &y;
+  constexpr const Y *q = dynamic_cast<const Y*>(p);
+}