[CodeGen] Migrate away from PointerUnion::dyn_cast (NFC) (#125456)

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 E to be nonnull.

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -2239,8 +2239,8 @@ CGCallee ItaniumCXXABI::getVirtualFunctionPointer(CodeGenFunction &CGF,
 llvm::Value *ItaniumCXXABI::EmitVirtualDestructorCall(
     CodeGenFunction &CGF, const CXXDestructorDecl *Dtor, CXXDtorType DtorType,
     Address This, DeleteOrMemberCallExpr E, llvm::CallBase **CallOrInvoke) {
-  auto *CE = E.dyn_cast<const CXXMemberCallExpr *>();
-  auto *D = E.dyn_cast<const CXXDeleteExpr *>();
+  auto *CE = dyn_cast<const CXXMemberCallExpr *>(E);
+  auto *D = dyn_cast<const CXXDeleteExpr *>(E);
   assert((CE != nullptr) ^ (D != nullptr));
   assert(CE == nullptr || CE->arg_begin() == CE->arg_end());
   assert(DtorType == Dtor_Deleting || DtorType == Dtor_Complete);