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_hir/src/hir.rs

@@ -732,6 +732,9 @@ pub struct Pat<'hir> {
     pub hir_id: HirId,
     pub kind: PatKind<'hir>,
     pub span: Span,
+    // Whether to use default binding modes.
+    // At present, this is false only for destructuring assignment.
+    pub default_binding_modes: bool,
 }
 
 impl Pat<'_> {
@@ -1680,6 +1683,9 @@ pub enum LocalSource {
     AsyncFn,
     /// A desugared `<expr>.await`.
     AwaitDesugar,
+    /// A desugared `expr = expr`, where the LHS is a tuple, struct or array.
+    /// The span is that of the `=` sign.
+    AssignDesugar(Span),
 }
 
 /// Hints at the original code for a `match _ { .. }`.