rustdoc: some fatal lexing errors are not buffered

[euclio]

Discovered while working on #56884.

For code like the following:

/// ```
/// "unterminated
/// ```
pub struct Foo;

rustdoc will fail to output any documentation for the struct and outputs a fatal error. I would expect the lexing error to be caught and emitted without stopping rendering.

[GuillaumeGomez]

Closing this issue since it has been fixed by #56884.

[euclio]

This wasn't fixed by #56884. Current output:

$ rustdoc +nightly test.rs
error: unterminated double quote string
 --> <doctest>:1:1
  |
1 | "unterminated
  | ^^^^^^^^^^^^^^

This is a problem with the lexer.

[GuillaumeGomez]

Ah my bad. Reopening.