make panictry! private to libsyntax

This commit completely removes usage of the `panictry!` macro from
outside libsyntax. The macro causes parse errors to be fatal, so using
it in libsyntax_ext caused parse failures *within* a syntax extension to
be fatal, which is probably not intended.

Furthermore, this commit adds spans to diagnostics emitted by empty
extensions if they were missing, à la #56491.

src/librustdoc/lib.rs

@@ -32,7 +32,7 @@ extern crate rustc_metadata;
 extern crate rustc_target;
 extern crate rustc_typeck;
 extern crate serialize;
-#[macro_use] extern crate syntax;
+extern crate syntax;
 extern crate syntax_pos;
 extern crate test as testing;
 #[macro_use] extern crate log;