Add tests for delegation generics
This PR adds tests from rust-lang/rust#151864 as discussed in this [comment](https://github.com/rust-lang/rust/pull/151864#issuecomment-3928053049). Part of rust-lang/rust#118212.
The majority of new tests are added in `mapping` folder, (`ast-hir-engine` folder in rust-lang/rust#151864), those tests test mapping between generic params of delegee and our generated function for delegation (that is why the name of the folder was changed, I think it better reflects what those tests testing). In each mapping test comments were added to each test case and one comment describing the goal of mapping tests was added at the top of each file.
Next, tests for defaults in generic params (`generic-params-defaults.rs`), params with the same name (`generic-params-same-names`), errors in providing user-specified generic args (`generics-gen-args-errors.rs`) and wrong signature of a generated function in impl trait case (`impl-trait-wrong-args-count.rs`, renamed from `wrong-args-count-ice.rs` in rust-lang/rust#151864). `generic-aux-pass.rs` test was not added, as all reuses in this test produce known ICE `DefId::expect_local DefId(..) isn't local` rust-lang/rust#143498 (which will be fixed in rust-lang/rust#151864, however it will not close mentioned issue, as synthetic generic params are not yet supported in new generics implementation).
r? @petrochenkov
Jonathan Brouwer
fd38ec4690