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_mir_build/src/thir/pattern/check_match.rs

@@ -69,6 +69,7 @@ impl<'tcx> Visitor<'tcx> for MatchVisitor<'_, 'tcx> {
             hir::LocalSource::ForLoopDesugar => ("`for` loop binding", None),
             hir::LocalSource::AsyncFn => ("async fn binding", None),
             hir::LocalSource::AwaitDesugar => ("`await` future binding", None),
+            hir::LocalSource::AssignDesugar(_) => ("destructuring assignment binding", None),
         };
         self.check_irrefutable(&loc.pat, msg, sp);
         self.check_patterns(&loc.pat);