[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#124883)

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>

This patch migrates the use of PointerUnion::dyn_cast to
dyn_cast_if_present because ShadowMapEntry::Add starts with:

  if (DeclOrVector.isNull()) {

implying that DeclOrVector is not guaranteed to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaCodeComplete.cpp

@@ -137,7 +137,7 @@ class ResultBuilder {
 
     ~ShadowMapEntry() {
       if (DeclIndexPairVector *Vec =
-              DeclOrVector.dyn_cast<DeclIndexPairVector *>()) {
+              dyn_cast_if_present<DeclIndexPairVector *>(DeclOrVector)) {
         delete Vec;
         DeclOrVector = ((NamedDecl *)nullptr);
       }