[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#125811)
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 Specialized to be nonnull. Note that if Specialized
were
null, dereferencing Tmpl would trigger a segfault.