Fix incorrect return type of emit_fatal

Co-authored-by: Giacomo Stevanato <giaco.stevanato@gmail.com>

compiler/rustc_session/src/session.rs

@@ -488,7 +488,7 @@ impl Session {
     ) -> DiagnosticBuilder<'a, !> {
         self.parse_sess.create_fatal(fatal)
     }
-    pub fn emit_fatal<'a>(&'a self, fatal: impl SessionDiagnostic<'a, !>) {
+    pub fn emit_fatal<'a>(&'a self, fatal: impl SessionDiagnostic<'a, !>) -> ! {
         self.parse_sess.emit_fatal(fatal)
     }
     #[inline]