[clang] [AST] Migrate away from PointerUnion::dyn_cast (NFC) (PR #122854)

[Nikita Popov via cfe-commits]

https://github.com/nikic commented:

> 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 Source to be nonnull.

You are still using dyn_cast_if_present :)

https://github.com/llvm/llvm-project/pull/122854