[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#123158)

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 AP to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaFunctionEffects.cpp

@@ -567,7 +567,7 @@ class Analyzer {
     while (!VerificationQueue.empty()) {
       const Decl *D = VerificationQueue.back();
       if (FuncAnalysisPtr AP = DeclAnalysis.lookup(D)) {
-        if (auto *Pending = AP.dyn_cast<PendingFunctionAnalysis *>()) {
+        if (auto *Pending = dyn_cast<PendingFunctionAnalysis *>(AP)) {
           // All children have been traversed; finish analysis.
           finishPendingAnalysis(D, Pending);
         }