DynamicAPInt is missing a conversion function back to APInt

Code builder/generator APIs work on APInt, not on DynamicAPInt. Once I do computations on DynamicAPInt values, I need to convert them back to the appropriately sized APInt. There's a constructor taking an APInt value, but not the other way around. There's a conversion function back to `int64_t`, but that won't work if I want to create an `int128_t` (or maybe 2 `int64_t` to initialize an array).

Right now I have to convert `DynamicAPInt` to a string and then from a string to another `APInt`. This is less than optimal. `DynamicAPInt` already stores an `APInt` internally so it'd be trivial for the class to just have a conversion function that copies such value.