[Serialization] Migrate away from PointerUnion::dyn_cast (NFC) (#124884)

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 DetailRecord to be nonnull.

Author: kazutakahirata

clang/lib/Serialization/ASTWriterStmt.cpp

@@ -475,7 +475,7 @@ addConstraintSatisfaction(ASTRecordWriter &Record,
   if (!Satisfaction.IsSatisfied) {
     Record.push_back(Satisfaction.NumRecords);
     for (const auto &DetailRecord : Satisfaction) {
-      auto *E = DetailRecord.dyn_cast<Expr *>();
+      auto *E = dyn_cast<Expr *>(DetailRecord);
       Record.push_back(/* IsDiagnostic */ E == nullptr);
       if (E)
         Record.AddStmt(E);