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/test/ui/macros/format-parse-errors.rs

@@ -0,0 +1,10 @@
+fn main() {
+    format!(); //~ ERROR requires at least a format string argument
+    format!(struct); //~ ERROR expected expression
+    format!("s", name =); //~ ERROR expected expression
+    format!("s", foo = struct); //~ ERROR expected expression
+    format!("s", struct); //~ ERROR expected expression
+
+    // This error should come after parsing errors to ensure they are non-fatal.
+    format!(123); //~ ERROR format argument must be a string literal
+}