Remove the global variables that store the discriminant IDs per variant

[kevina]

Consider removing global variables that store the discriminant IDs per variant as they are likely not really needed.

See pull request #1397.

[kevina]

One of the reasons I am hesitant about doing this is that it may be useful to have a an actual variable if it is necessary to take a reference of the tag literal. Without out a global variable to refer to a temporary will need to be allocated on the stack.

Then again, I could be wrong, I am basing my knowledge with how C++ would work, Rust could be different.

[graydon]

No, I think you're right. Closing this as I think reference-to-constant-enum is something that could sometimes happen.