Rollup merge of #68073 - Centril:fix-68062, r=estebank

expect `fn` after `const unsafe` / `const extern`

Fixes #68062

r? @estebank @petrochenkov
cc @Aaron1011

src/librustc_parse/parser/item.rs

@@ -140,7 +140,7 @@ impl<'a> Parser<'a> {
                     self.sess.gated_spans.gate(sym::const_extern_fn, lo.to(self.token.span));
                 }
                 let ext = self.parse_extern()?;
-                self.bump(); // `fn`
+                self.expect_keyword(kw::Fn)?;
 
                 let header = FnHeader {
                     unsafety,