Long error diagnostics should not show compiletest/highlighting metadata in text mode

[Manishearth]

e.g.

$ rustc --explain E0413
A declaration shadows an enum variant or unit-like struct in scope. Example of
erroneous code:

```compile_fail
struct Foo;
...

Stuff like ````compile_fail` shouldn't show, just the backticks.

cc @GuillaumeGomez @steveklabnik

[GuillaumeGomez]

I'm on it!