rustc_span: Provide a reserved identifier check for a specific edition
rustc_span: Provide a reserved identifier check for a specific edition

Edition evaluation is kept lazy because it may be expensive.

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