Detect when `for` loop was written when a `while let` was meant

Current output:

error[E0382]: borrow of moved value: `iter`
    --> compiler/rustc_resolve/src/diagnostics.rs:1797:50
     |
1786 |             let mut iter = struct_expr.fields.iter().peekable();
     |                 -------- move occurs because `iter` has type `Peekable<std::slice::Iter<'_, rustc_ast::ExprField>>`, which does not implement the `Copy` trait
1787 |             let mut prev: Option<Span> = None;
1788 |             for field in iter {
     |                          ---- `iter` moved due to this implicit call to `.into_iter()`
...
1797 |                                     match (prev, iter.peek()) {
     |                                                  ^^^^ value borrowed here after move
     |
note: `std::iter::IntoIterator::into_iter` takes ownership of the receiver `self`, which moves `iter`
    --> /home/gh-estebank/rust/library/core/src/iter/traits/collect.rs:313:18
     |
313  |     fn into_iter(self) -> Self::IntoIter;
     |                  ^^^^

It should suggest while let Some(field) = iter { on line 1788.

Start at compiler/rustc_resolve/src/diagnostics.rs:1788, where the for field in iter loop produces the shown moved-value diagnostic. Trace the surrounding diagnostic handling and verify that this case suggests while let Some(field) = iter {. Done means the compiler emits that suggestion for the reported pattern.