Merge #6376

6376: Avoid impls_fnonce to return true when the trait solving is ambiguous  r=flodiebold a=GrayJack

This PR should fix #6375

This adds a variation of `method_resolution::implements_trait` called `method_resolution::implements_trait_unique`, that only returns true when the trait solving is unique, and also change `impls_fnonce` to use the later instead.

I also added a test just to be sure.

Co-authored-by: GrayJack <gr41.j4ck@gmail.com>