[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#124882)

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 It->second to be nonnull.
getSingleDynTypedNodeFromParentMap ends with a deference of U.

Author: kazutakahirata

clang/lib/AST/ParentMapContext.cpp

@@ -117,7 +117,7 @@ class ParentMapContext::ParentMap {
     if (I == Map.end()) {
       return llvm::ArrayRef<DynTypedNode>();
     }
-    if (const auto *V = I->second.template dyn_cast<ParentVector *>()) {
+    if (const auto *V = dyn_cast<ParentVector *>(I->second)) {
       return V->view();
     }
     return getSingleDynTypedNodeFromParentMap(I->second);
@@ -268,9 +268,9 @@ class ParentMapContext::ParentMap {
       auto It = PointerParents.find(E);
       if (It == PointerParents.end())
         break;
-      const auto *S = It->second.dyn_cast<const Stmt *>();
+      const auto *S = dyn_cast<const Stmt *>(It->second);
       if (!S) {
-        if (auto *Vec = It->second.dyn_cast<ParentVector *>())
+        if (auto *Vec = dyn_cast<ParentVector *>(It->second))
           return Vec->view();
         return getSingleDynTypedNodeFromParentMap(It->second);
       }