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

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 U to be nonnull.

Author: kazutakahirata

clang/lib/AST/ParentMapContext.cpp

@@ -103,9 +103,9 @@ class ParentMapContext::ParentMap {
 
   static DynTypedNode
   getSingleDynTypedNodeFromParentMap(ParentMapPointers::mapped_type U) {
-    if (const auto *D = U.dyn_cast<const Decl *>())
+    if (const auto *D = dyn_cast<const Decl *>(U))
       return DynTypedNode::create(*D);
-    if (const auto *S = U.dyn_cast<const Stmt *>())
+    if (const auto *S = dyn_cast<const Stmt *>(U))
       return DynTypedNode::create(*S);
     return *cast<DynTypedNode *>(U);
   }