deriving codegen: add intrinsic to map enum instance to its variant index

[pnkfelix]

Spawned off of #15503 (comment)

The other piece of the story there would be speeding up the mapping from the value to its discriminant index. Currently I am just emitting that code in place as another match statement, but in principle that could (and should?) be replaced by a compiler intrinsic, as discussed here: #15375 (comment)

This is already exposed in the TyVisitor intrinsic: http://static.rust-lang.org/doc/master/core/intrinsics/trait.TyVisitor.html#tymethod.visit_enter_enum. Obviously, using that could be undesirable from a code bloat perspective so a separate intrinsic can still be a good idea.

[pnkfelix]

The semantics of this intrinsic need to be consistent with whatever we decide to do for #15523

[emberian]

For my usecase, I don't really care about order, just equality, and this would let me compare token types without a very large match (that compiles to pretty poor code) and without transmuting to a fragile representation.

[pnkfelix]

do note also huon's point here: #15375 (comment)

[durka]

Should this still be open? What's the point of a variant index intrinsic?

[pnkfelix]

Due to huon's point from #15375 that the ordering (in terms of the value used for the discriminant) might change over time with changes to the compiler, plus the fact that the comments here indicate that the important thing is equality comparisons, not an ordering nor assignment to integer values, I am just going to close this.