[llvm][ADT] Allow returning `std::nullopt` in TypeSwitch

Returning `std::nullopt` from the case of a `TypeSwitch` yields broken results, by either falling through to another case, or falling of the switch entirely and hitting an assertion. This is simply due to the use of `operator=` of what is now `std::optional`, which has an overload specifically for `std::nullopt`, causing the internal optional, used for the `TypeSwitch` result to be reset, instead of a value being constructed from the `std::nullopt`.

The fix is to simply use the `emplace` method of `std::optional`, causing a value to always be constructed from the value returned by the case function.

Differential Revision: https://reviews.llvm.org/D140265

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