[ADT] Support `.Default` with `nullptr` and `nullopt` values in TypeSwitch (#165724)

In the previous implementation, this would fail for cases like
`TypeSwitch<T*, std::optional<U>>` because `std::nullopt` does not match
`ResultT` exactly and the overload for callable types would be selected.

Add new overloads that support `nullptr` and `std::nullopt`. These can
be added alongside generic callables because we wouldn't want to call
any 'null' function refs anyway.

I selected the `nullptr` and `nullopt` specializations because how often
they appear in the codebase -- currently, you will see lots of code like
`.Default(std::optional<T>())` that can be simplified with this patch.

0 files changed, 0 insertions, 0 deletions