[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#122855)

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 DeclOrVector to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaCodeComplete.cpp

@@ -121,8 +121,7 @@ class ResultBuilder {
         return;
       }
 
-      if (const NamedDecl *PrevND =
-              DeclOrVector.dyn_cast<const NamedDecl *>()) {
+      if (const NamedDecl *PrevND = dyn_cast<const NamedDecl *>(DeclOrVector)) {
         // 1 -> 2 elements: create the vector of results and push in the
         // existing declaration.
         DeclIndexPairVector *Vec = new DeclIndexPairVector;
@@ -702,7 +701,7 @@ ResultBuilder::ShadowMapEntry::begin() const {
   if (DeclOrVector.isNull())
     return iterator();
 
-  if (const NamedDecl *ND = DeclOrVector.dyn_cast<const NamedDecl *>())
+  if (const NamedDecl *ND = dyn_cast<const NamedDecl *>(DeclOrVector))
     return iterator(ND, SingleDeclIndex);
 
   return iterator(cast<DeclIndexPairVector *>(DeclOrVector)->begin());