Simplify `reindent_multiline()` signature (#14101)

- `reindent_multiline()` always returns the result of
`reindent_multiline_inner()` which returns a `String`. Make
`reindent_multiline()` return a `String` as well, instead of a
systematically owned `Cow<'_, str>`.
- There is no reason for `reindent_multiline()` to force a caller to
build a `Cow<'_, str>` instead of passing a `&str` directly, especially
considering that a `String` will always be returned.

Also, both the input parameter and return value (of type `Cow<'_, str>`)
shared the same (elided) lifetime for no reason: this worked only
because the result was always the `Cow::Owned` variant which is
compatible with any lifetime.

As a consequence, the signature changes from:

```rust
fn reindent_multiline(s: Cow<'_, str>, …) -> Cow<'_, str> { … }
```

to

```rust
fn reindent_multiline(s: &str, …) -> String { … }
```

changelog: none

0 files changed, 0 insertions, 0 deletions