Simplify attribute handling in rustc_ast_lowering

Given that attributes is stored in a separate BTreeMap, it's not necessary
to pass it in when constructing `hir::Expr`. We can just construct
`hir::Expr` and then call `self.lower_attrs` later if it needs attributes.

As most desugaring code don't use attributes, this allows some code cleanup.

compiler/rustc_ast_lowering/src/lib.rs

@@ -2291,7 +2291,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// has no attributes and is not targeted by a `break`.
     fn lower_block_expr(&mut self, b: &Block) -> hir::Expr<'hir> {
         let block = self.lower_block(b, false);
-        self.expr_block(block, AttrVec::new())
+        self.expr_block(block)
     }
 
     fn lower_array_length(&mut self, c: &AnonConst) -> hir::ArrayLen {