Simplify attribute handling in parse_bottom_expr.

`Parser::parse_bottom_expr` currently constructs an empty `attrs` and
then passes it to a large number of other functions. This makes the code
harder to read than it should be, because it's not clear that many
`attrs` arguments are always empty.

This commit removes `attrs` and the passing, simplifying a lot of
functions. The commit also renames `Parser::mk_expr` (which takes an
`attrs` argument) as `mk_expr_with_attrs`, and introduces a new
`mk_expr` which creates an expression with no attributes, which is the
more common case.

compiler/rustc_parse/src/parser/mod.rs

@@ -1116,10 +1116,14 @@ impl<'a> Parser<'a> {
         let (attrs, blk) = self.parse_inner_attrs_and_block()?;
         let anon_const = AnonConst {
             id: DUMMY_NODE_ID,
-            value: self.mk_expr(blk.span, ExprKind::Block(blk, None), AttrVec::new()),
+            value: self.mk_expr(blk.span, ExprKind::Block(blk, None)),
         };
         let blk_span = anon_const.value.span;
-        Ok(self.mk_expr(span.to(blk_span), ExprKind::ConstBlock(anon_const), AttrVec::from(attrs)))
+        Ok(self.mk_expr_with_attrs(
+            span.to(blk_span),
+            ExprKind::ConstBlock(anon_const),
+            AttrVec::from(attrs),
+        ))
     }
 
     /// Parses mutability (`mut` or nothing).