concat_idents cannot be used in pattern position

[ExpHP]

• 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.

[cyrgani]

concat_idents is being removed in #142704. The new macro_metavar_expr_concat feature replaces it and can be used in pattern positions as well.