Hack: do not expand arg <expr> within ensure_not_fmt_string_literal.

An attempt to side-step the stability gates that are claiming
that ensure_not_fmt_string_literal is expanding into uses of
format_args internals.

This attempt however did not work; this commit is only here for
historical purposs while I ask others for advice.

Here is the stack trace of the relevant error here:

```
---- [run-fail] run-fail/explicit-panic-msg.rs stdout ----

error: compilation failed!
status: exit code: 101
command: x86_64-apple-darwin/stage1/bin/rustc /Users/fklock/Dev/Mozilla/rust-panic/src/test/run-fail/explicit-panic-msg.rs -L x86_64-apple-darwin/test/run-fail/ --target=x86_64-apple-darwin -L x86_64-apple-darwin/test/run-fail/explicit-panic-msg.stage1-x86_64-apple-darwinlibaux -C prefer-dynamic -o x86_64-apple-darwin/test/run-fail/explicit-panic-msg.stage1-x86_64-apple-darwin --cfg rtopt --cfg debug -L x86_64-apple-darwin/rt
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
<std macros>:2:26: 2:57 error: use of unstable library feature 'core': internal to format_args!
<std macros>:2 $ crate:: fmt:: format ( format_args ! ( $ ( $ arg ) * ) ) )
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of __unstable_rustc_ensure_not_fmt_string_literal!
<std macros>:4:1: 4:78 note: expansion site
<std macros>:1:1: 12:23 note: in expansion of panic!
/Users/fklock/Dev/Mozilla/rust-panic/src/test/run-fail/explicit-panic-msg.rs:18:5: 18:37 note: expansion site
<std macros>:2:26: 2:57 help: add #![feature(core)] to the crate attributes to enable
<std macros>:2:26: 2:57 error: use of unstable library feature 'core': internal to format_args!
<std macros>:2 $ crate:: fmt:: format ( format_args ! ( $ ( $ arg ) * ) ) )
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of __unstable_rustc_ensure_not_fmt_string_literal!
<std macros>:4:1: 4:78 note: expansion site
<std macros>:1:1: 12:23 note: in expansion of panic!
/Users/fklock/Dev/Mozilla/rust-panic/src/test/run-fail/explicit-panic-msg.rs:18:5: 18:37 note: expansion site
<std macros>:2:26: 2:57 help: add #![feature(core)] to the crate attributes to enable
error: aborting due to 2 previous errors

------------------------------------------

thread '[run-fail] run-fail/explicit-panic-msg.rs' panicked at 'explicit panic', /Users/fklock/Dev/Mozilla/rust-panic/src/compiletest/runtest.rs:1499

failures:
    [run-fail] run-fail/explicit-panic-msg.rs
```

Author: pnkfelix

src/libsyntax/ext/format.rs

@@ -651,7 +651,8 @@ pub fn ensure_not_fmt_string_literal<'cx>(cx: &'cx mut ExtCtxt,
     if p.token == token::Eof { return takes_two_args(cx, "given 1"); }
     if !panictry!(p.eat(&token::Comma)) { return takes_two_args(cx, "comma-separated"); }
     if p.token == token::Eof { return takes_two_args(cx, "given 1"); }
-    let arg2 = cx.expander().fold_expr(p.parse_expr());
+    // do not expand the `<expr>`; just inspect it as a black box.
+    let arg2 = p.parse_expr();
     if p.token != token::Eof {
         takes_two_args(cx, "given too many");
         // (but do not return; handle two provided, nonetheless)