fix: correct suggestion for significant_drop_in_scrutinee in expressions

[vishruth-thimmaiah]

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:

    let mutex = Mutex::new(State {});
    let _ = match mutex.lock().unwrap().foo() {
        true => 0,
        false => 1,
    };

would suggest:

let _ = let value = mutex.lock().unwrap().foo();
match value {

With this PR, it now suggests:

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

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[y21]

I think this could still break in some cases, e.g. if the match expression is an argument to a function call that is broken up into multiple lines like:

foo(
  arg1,
  arg2,
  match .. { // can't put the `let` here
    ..
  }
);

We could do more precise things like iterating parent nodes with cx.tcx.hir().parent_iter(expr.hir_id) until we see a Node::Stmt or Node::Block which should give us a place to put the let statement that is always syntactically valid at least, but even that wouldn't always compile (and it might also be a bit much for a suggestion that is marked as MaybeIncorrect anyways). It does fix the linked reproducer at least, so LGTM