Improve formatting of closure capture migration suggestion for multi-line closures.

[m-ou-se]

Fixes #87952

Before:

help: add a dummy let to cause `a` to be fully captured
  |
5 ~     let _ = || { let _ = &a; 
6 +         dbg!(a.0);
7 ~     };
  |

After:

help: add a dummy let to cause `a` to be fully captured
  |
5 ~     let _ = || {
6 +         let _ = &a;
7 +         dbg!(a.0);
8 ~     };
  |

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

@bors r+

[bors]

📌 Commit 31fcc94 has been approved by petrochenkov