[Serialization] Migrate away from PointerUnion::dyn_cast (NFC) (#124884) (#125024)

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 Subject to be nonnull.

Author: kazutakahirata

clang/lib/Serialization/ASTWriter.cpp

@@ -5359,7 +5359,7 @@ ASTWriter::WriteAST(llvm::PointerUnion<Sema *, Preprocessor *> Subject,
   llvm::TimeTraceScope scope("WriteAST", OutputFile);
   WritingAST = true;
 
-  Sema *SemaPtr = Subject.dyn_cast<Sema *>();
+  Sema *SemaPtr = dyn_cast<Sema *>(Subject);
   Preprocessor &PPRef =
       SemaPtr ? SemaPtr->getPreprocessor() : *cast<Preprocessor *>(Subject);