Make rustc_attr::parse_version pub

compiler/rustc_attr/src/builtin.rs

@@ -574,7 +574,7 @@ fn gate_cfg(gated_cfg: &GatedCfg, cfg_span: Span, sess: &Session, features: &Fea
 /// Parse a rustc version number written inside string literal in an attribute,
 /// like appears in `since = "1.0.0"`. Suffixes like "-dev" and "-nightly" are
 /// not accepted in this position, unlike when parsing CFG_RELEASE.
-fn parse_version(s: Symbol) -> Option<RustcVersion> {
+pub fn parse_version(s: Symbol) -> Option<RustcVersion> {
     let mut components = s.as_str().split('-');
     let d = components.next()?;
     if components.next().is_some() {