[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#125630)

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 *Found to be nonnull.  Note that if *Found were
null, cast<VarDecl>(TransformedDecl) would trigger an assertion error.

Author: kazutakahirata

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -2438,7 +2438,7 @@ TemplateInstantiator::TransformFunctionParmPackRefExpr(DeclRefExpr *E,
   assert(Found && "no instantiation for parameter pack");
 
   Decl *TransformedDecl;
-  if (DeclArgumentPack *Pack = Found->dyn_cast<DeclArgumentPack *>()) {
+  if (DeclArgumentPack *Pack = dyn_cast<DeclArgumentPack *>(*Found)) {
     // If this is a reference to a function parameter pack which we can
     // substitute but can't yet expand, build a FunctionParmPackExpr for it.
     if (getSema().ArgumentPackSubstitutionIndex == -1) {