Rollup merge of #78748 - fanzier:tuple-assignment, r=petrochenkov

Implement destructuring assignment for tuples

This is the first step towards implementing destructuring assignment (RFC: https://github.com/rust-lang/rfcs/pull/2909, tracking issue: #71126). This PR is the first part of #71156, which was split up to allow for easier review.

Quick summary: This change allows destructuring the LHS of an assignment if it's a (possibly nested) tuple.
It is implemented via a desugaring (AST -> HIR lowering) as follows:
```rust
(a,b) = (1,2)
```
... becomes ...
```rust
{
  let (lhs0,lhs1) = (1,2);
  a = lhs0;
  b = lhs1;
}
```

Thanks to `@varkor` who helped with the implementation, particularly around default binding modes.

r? `@petrochenkov`

compiler/rustc_span/src/symbol.rs

@@ -434,6 +434,7 @@ symbols! {
         deref_mut,
         deref_target,
         derive,
+        destructuring_assignment,
         diagnostic,
         direct,
         discriminant_kind,