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````

compiler/rustc_parse/src/parser/expr.rs

@@ -1089,6 +1089,9 @@ impl<'a> Parser<'a> {
             self.parse_yield_expr(attrs)
         } else if self.eat_keyword(kw::Let) {
             self.parse_let_expr(attrs)
+        } else if self.eat_keyword(kw::Underscore) {
+            self.sess.gated_spans.gate(sym::destructuring_assignment, self.prev_token.span);
+            Ok(self.mk_expr(self.prev_token.span, ExprKind::Underscore, attrs))
         } else if !self.unclosed_delims.is_empty() && self.check(&token::Semi) {
             // Don't complain about bare semicolons after unclosed braces
             // recovery in order to keep the error count down. Fixing the