concat_idents cannot be used in pattern position

* The three places where an ident-returning macro can be used are patterns, types, and expressions.
* The two places where `concat_idents` can be used are types and expressions.
* **_Hmmmm._**

I looked around previous issues and PRs related to `concat_idents` and the word "pattern" never even comes up.  This seems to be an accidental omission.