[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (llvm#125023)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect Entry to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaDecl.cpp

@@ -19966,7 +19966,7 @@ static void CheckForDuplicateEnumValues(Sema &S, ArrayRef<Decl *> Elements,
       continue;
 
     DeclOrVector& Entry = Iter->second;
-    if (EnumConstantDecl *D = Entry.dyn_cast<EnumConstantDecl*>()) {
+    if (EnumConstantDecl *D = dyn_cast<EnumConstantDecl *>(Entry)) {
       // Ensure constants are different.
       if (D == ECD)
         continue;