Rollup merge of #49540 - bjorn3:fix_miri_discriminant, r=oli-obk
Rollup merge of #49540 - bjorn3:fix_miri_discriminant, r=oli-obk

Fix miri Discriminant() for non-ADT

Fixes #49327

GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust