[`unconditional_recursion`]: compare by `Ty`s instead of `DefId`s

Fixes rust-lang#12154
Fixes rust-lang#12181 (this was later edited in, so the rest of the description refers to the first linked issue)

Before this change, the lint would work with `DefId`s and use those to compare types. This PR changes it to compare types directly. It fixes the linked issue, but also other false positives I found in a lintcheck run. For example, one of the issues is that some types don't have `DefId`s (primitives, references, etc., leading to possible FNs), and the helper function used to extract a `DefId` didn't handle type parameters.

Another issue was that the lint would use `.peel_refs()` in a few places where that could lead to false positives (one such FP was in the `http` crate). See the doc comment on one of the added functions and also the test case for what I mean.

The code in the linked issue was linted because the receiver type is `T` (a `ty::Param`), which was not handled in `get_ty_def_id` and returned `None`, so this wouldn't actually *get* to comparing `self_arg != ty_id` here, and skip the early-return:
https://github.com/rust-lang/rust-clippy/blob/70573af31eb9b8431c2e7923325c82ba0304cbb2/clippy_lints/src/unconditional_recursion.rs#L171-L178

This alone could be fixed by doing something like `&& get_ty_def_id(ty).map_or(true, |ty_id)| self_arg != ty_id)`, but we don't really need to work with `DefId`s in the first place, I don't think.

changelog: [`unconditional_recursion`]: avoid linting when the other comparison type is a type parameter