Add new useless_concat lint

[GuillaumeGomez]

Fixes #13793.

Interestingly enough, to actually check that the macro call has at least two arguments, we need to use the rust lexer after getting the original source code snippet.

changelog: Add new useless_concat lint

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[GuillaumeGomez]

Finally was able to only trigger the lint when appropriate.

[GuillaumeGomez]

r? clippy