Fix suggestion for attempting to define a string with single quotes

[beetrees]

Currently attempting to compile fn main() { let _ = '\\"'; } will result in the following error message:

error: character literal may only contain one codepoint
 --> src/main.rs:1:21
  |
1 | fn main() { let _ = '\\"'; }
  |                     ^^^^^
  |
help: if you meant to write a `str` literal, use double quotes
  |
1 | fn main() { let _ = "\\""; }
  |                     ~~~~~

The suggestion is invalid as it fails to escape the ". This PR fixes the suggestion so that it now reads:

help: if you meant to write a `str` literal, use double quotes
  |
1 | fn main() { let _ = "\\\""; }
  |                     ~~~~~~

The relevant test is also updated to ensure that this does not regress in future.

[rustbot]

r? @davidtwco

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[davidtwco]

@bors r+

[bors]

📌 Commit 072d8c8 has been approved by davidtwco

It is now in the queue for this repository.