[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#124447)

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 AnyFunc to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaAPINotes.cpp

@@ -478,7 +478,7 @@ static void ProcessAPINotes(Sema &S, FunctionOrMethod AnyFunc,
                             const api_notes::FunctionInfo &Info,
                             VersionedInfoMetadata Metadata) {
   // Find the declaration itself.
-  FunctionDecl *FD = AnyFunc.dyn_cast<FunctionDecl *>();
+  FunctionDecl *FD = dyn_cast<FunctionDecl *>(AnyFunc);
   Decl *D = FD;
   ObjCMethodDecl *MD = nullptr;
   if (!D) {