Rollup merge of #44552 - durka:patch-42, r=alexcrichton
Rollup merge of #44552 - durka:patch-42, r=alexcrichton

update "since" for discriminant_value

It's [going](https://github.com/rust-lang/rust/pull/44263#issuecomment-329257597) to be backported to beta.

r? @alexcrichton

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