Rollup merge of #79016 - fanzier:underscore-expressions, r=petrochenkov

Make `_` an expression, to discard values in destructuring assignments

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

With this PR, an underscore `_` is parsed as an expression but is allowed *only* on the left-hand side of a destructuring assignment. There it simply discards a value, similarly to the wildcard `_` in patterns. For instance,
```rust
(a, _) = (1, 2)
```
will simply assign 1 to `a` and discard the 2. Note that for consistency,
```
_ = foo
```
is also allowed and equivalent to just `foo`.

Thanks to ````@varkor```` who helped with the implementation, particularly around pre-expansion gating.

r? ````@petrochenkov````

clippy_lints/src/utils/sugg.rs

@@ -170,6 +170,7 @@ impl<'a> Sugg<'a> {
             | ast::ExprKind::MacCall(..)
             | ast::ExprKind::MethodCall(..)
             | ast::ExprKind::Paren(..)
+            | ast::ExprKind::Underscore
             | ast::ExprKind::Path(..)
             | ast::ExprKind::Repeat(..)
             | ast::ExprKind::Ret(..)