Add support for postfix yield expressions

We had a discussion[1] today about whether postfix yield would make sense.
It's easy enough to support both in the parser, so we might as well have
both and see how people use it while the feature is experimental.

[1]: https://rust-lang.zulipchat.com/#narrow/channel/481571-t-lang.2Fgen/topic/postfix-yield/with/505231568

compiler/rustc_parse/src/parser/expr.rs

@@ -1310,6 +1310,13 @@ impl<'a> Parser<'a> {
             return self.parse_match_block(lo, match_span, self_arg, MatchKind::Postfix);
         }
 
+        // Post-fix yield
+        if self.eat_keyword(exp!(Yield)) {
+            let yield_span = self.prev_token.span;
+            self.psess.gated_spans.gate(sym::yield_expr, yield_span);
+            return Ok(self.mk_expr(yield_span, ExprKind::Yield(Some(self_arg))));
+        }
+
         let fn_span_lo = self.token.span;
         let mut seg = self.parse_path_segment(PathStyle::Expr, None)?;
         self.check_trailing_angle_brackets(&seg, &[exp!(OpenParen)]);