pattern analysis: Require enum indices to be contiguous
We had a cfg-hack to allow rust-analyzer to use non-contiguous indices for its enum variants. Unfortunately this no longer works if r-a uses the in-tree version of the crate.
This PR removes the hack, and on the r-a side we'll have to use contiguous indices but that's not too hard.
r? `@compiler-errors`
f04d068adc