Unconditionally capture tokens for attributes.

This allows us to avoid synthesizing tokens in `prepend_attr`, since we
have the original tokens available.

We still need to synthesize tokens when expanding `cfg_attr`,
but this is an unavoidable consequence of the syntax of `cfg_attr` -
the user does not supply the `#` and `[]` tokens that a `cfg_attr`
expands to.

compiler/rustc_expand/src/expand.rs

@@ -1785,6 +1785,7 @@ impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
                 span: at.span,
                 id: at.id,
                 style: at.style,
+                tokens: None,
             };
         } else {
             noop_visit_attribute(at, self)