fix: correct suggestion for significant_drop_in_scrutinee in expressions (#14019)

This PR fixes an issue with the `significant_drop_in_scrutinee`, where
the lint generates invalid Rust syntax when suggesting fixes for match
expressions that are part of larger expressions, such as in assignment
contexts. For example:

```rust
    let mutex = Mutex::new(State {});
    let _ = match mutex.lock().unwrap().foo() {
        true => 0,
        false => 1,
    };
```
would suggest:
```rust
let _ = let value = mutex.lock().unwrap().foo();
match value {
```
With this PR, it now suggests:
```rust
let value = mutex.lock().unwrap().foo();
let _ = match value {
```

closes: #13986

changelog: [`significant_drop_in_scrutinee`] Fix incorrect suggestion
for `significant_drop_in_scrutinee` lint in expression context

clippy_lints/src/matches/significant_drop_in_scrutinee.rs

@@ -2,7 +2,7 @@ use std::ops::ControlFlow;
 
 use crate::FxHashSet;
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::source::{indent_of, snippet};
+use clippy_utils::source::{first_line_of_span, indent_of, snippet};
 use clippy_utils::ty::{for_each_top_level_late_bound_region, is_copy};
 use clippy_utils::{get_attr, is_lint_allowed};
 use itertools::Itertools;
@@ -152,7 +152,7 @@ fn set_suggestion<'tcx>(diag: &mut Diag<'_, ()>, cx: &LateContext<'tcx>, expr: &
     diag.multipart_suggestion(
         suggestion_message,
         vec![
-            (expr.span.shrink_to_lo(), replacement),
+            (first_line_of_span(cx, expr.span).shrink_to_lo(), replacement),
             (found.found_span, scrutinee_replacement),
         ],
         Applicability::MaybeIncorrect,