jit_wrapper_generator should generate C++ version of DSLX enums

It would be nice if https://github.com/google/xls/blob/main/xls/jit/jit_wrapper_generator.cc generate C++ enums (or constexpr) from DSLX enum definitions.

Ex:
```
enum Opcode : u3 {
FIRE_THE_MISSILES = 0,
BE_TIRED = 1,
TAKE_A_NAP = 2,
}
```
Could generate something like:
```
enum Opcode {
FIRE_THE_MISSILES = 0,
BE_TIRED = 1,
TAKE_A_NAP = 2,
};

constexpr size_t OPCODE_ENUM_BITSIZE = 3;
```