[clang] [CodeGen] Migrate away from PointerUnion::dyn_cast (NFC) (PR #122653)

[Nikita Popov via cfe-commits]

nikic wrote:

> 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 Prototype.P to be nonnull.

This comment doesn't seem to match the PR :)

https://github.com/llvm/llvm-project/pull/122653