Rollup merge of #142671 - davidtwco:no-default-bounds-attr, r=lcnr

add #![rustc_no_implicit_bounds]

Follow-up from rust-lang/rust#137944.

Adds a new `rustc_attrs` attribute that stops rustc from adding any default bounds. Useful for tests where default bounds just add noise and make debugging harder.

After reviewing all tests with `?Sized`, these tests seem like they could probably benefit from `#![rustc_no_implicit_bounds]`.

- Skipping most of `tests/ui/unsized` as these seem to want to test `?Sized`
- Skipping tests that used `Box<T>` because it's still bound by `T: MetaSized`
- Skipping parsing or other tests that cared about `?Sized` syntactically
- Skipping tests for `derive(CoercePointee)` because this appears to check that the pointee type is relaxed with `?Sized` explicitly

r? `@lcnr`

Author: GuillaumeGomez