rollup merge of #22048: LeoTestard/impl-patterns-used

The live code analysis only visited the function's body when visiting a
method, and not the FnDecl and the generics, resulting in code to be
incorrectly marked as unused when it only appeared in the generics, the
arguments, or the return type, whereas the same code in non-method
functions was correctly detected as used. Fixes #20343.

Originally I just added a call to `walk_generics` and `walk_fndecl` alongside `walk_block` but then I noticed the `walk_method_helper` function did pretty much the same thing. The only difference is that it also calls `visit_mac`, but since this is not going to happen at this stage, I think it's ok. However let me know if this was not the right thing to do.

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