[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#124229)

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 UPP.first to be nonnull.

Author: kazutakahirata

clang/include/clang/Sema/SemaInternal.h

@@ -72,7 +72,7 @@ inline std::pair<unsigned, unsigned> getDepthAndIndex(const NamedDecl *ND) {
 /// Retrieve the depth and index of an unexpanded parameter pack.
 inline std::pair<unsigned, unsigned>
 getDepthAndIndex(UnexpandedParameterPack UPP) {
-  if (const auto *TTP = UPP.first.dyn_cast<const TemplateTypeParmType *>())
+  if (const auto *TTP = dyn_cast<const TemplateTypeParmType *>(UPP.first))
     return std::make_pair(TTP->getDepth(), TTP->getIndex());
 
   return getDepthAndIndex(cast<NamedDecl *>(UPP.first));