[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#123012)

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 P to be nonnull.

Author: kazutakahirata

clang/lib/AST/ASTImporter.cpp

@@ -6376,7 +6376,7 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateSpecializationDecl(
   D2->setTemplateSpecializationKind(D->getTemplateSpecializationKind());
 
   if (auto P = D->getInstantiatedFrom()) {
-    if (auto *CTD = P.dyn_cast<ClassTemplateDecl *>()) {
+    if (auto *CTD = dyn_cast<ClassTemplateDecl *>(P)) {
       if (auto CTDorErr = import(CTD))
         D2->setInstantiationOf(*CTDorErr);
     } else {