Arbitrary self types v2: deshadowing probe

This is the first part of a series of commits which impact the
"deshadowing detection" in the arbitrary self types v2 RFC.

This commit should not have any functional changes, but may impact
performance. Subsequent commits add back the performance, and add error
checking to this new code such that it has a functional effect.

Rust prioritizes method candidates in this order:
1. By value;
2. By reference;
3. By mutable reference;
4. By const ptr.
5. By reborrowed pin.

Previously, if a suitable candidate was found in one of these earlier
categories, Rust wouldn't even move onto probing the other categories.

As part of the arbitrary self types work, we're going to need to change
that - even if we choose a method from one of the earlier categories, we
will sometimes need to probe later categories to search for methods that
we may be shadowing.

This commit adds those extra searches for shadowing, but it does not yet
produce an error when such shadowing problems are found. That will come
in a subsequent commit, by filling out the 'check_for_shadowing'
method.

This commit contains a naive approach to detecting these shadowing
problems, which shows what we've functionally looking to do. However,
it's too slow. The performance of this approach was explored in this
PR:
https://github.com/rust-lang/rust/pull/127812#issuecomment-2236911900
Subsequent commits will improve the speed of the search.

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