Rollup merge of #71494 - flip1995:while_let_span, r=petrochenkov

Fix span of while (let) expressions after lowering

Credit goes to @alex-700 who found this while trying to fix a suggestion in Clippy.

While `if`, `try`, `for` and `await` expressions get the span of the original expression when desugared, `while` loops got the span of the scrutinee, which lead to weird code, when building the suggestion, that randomly worked: https://github.com/rust-lang/rust-clippy/pull/5511/files#diff-df4e9d2bf840a5f2e3b580bef73da3bcR106-R108

I'm wondering, if `DesugaringKind` should get a variant `WhileLoop` and instead of using the span of the `ast::ExprKind::While` expr directly, a new span with `self.mark_span_with_reason` should be used, like it is done with `for` loops.

There was some fallout, but I think that is acceptable. If not, I need some help to find out where this can be fixed.

src/test/ui/block-result/block-must-not-have-result-while.stderr

@@ -9,8 +9,14 @@ LL |     while true {
 error[E0308]: mismatched types
   --> $DIR/block-must-not-have-result-while.rs:3:9
    |
-LL |         true
-   |         ^^^^ expected `()`, found `bool`
+LL | /     while true {
+LL | |         true
+   | |         ^^^^ expected `()`, found `bool`
+LL | |
+LL | |     }
+   | |     -- help: consider using a semicolon here
+   | |_____|
+   |       expected this to be `()`
 
 error: aborting due to previous error; 1 warning emitted