go through uses of `DefineOpaqueTypes::No` and either document or change them

[oli-obk]

This issue can be resolved in individual steps (each application of all steps should be their own commit!):

1. flip a single usage of DefineOpauqeTypes::No to Yes
2. see what tests fail, document the site by mentioning the test in a comment, revert to ::No, and open a PR.
3. If no tests fail, still open a PR and assign to @oli-obk or @compiler-errors and leave a comment that there's no test exercising it. We'll try to come up with a test and/or run crater to find examples.

[goldpanth3r]

Hey @oli-obk! I would like to try to contribute to this. Could you explain Step 1:flip a site to Yes? I tried to search for site and DefineOpaqueTypes::No in the codebase, but couldn't learn much.

[matthiaskrgr]

I think it means to just search for DefineOpaqueTypes::No for example:

compiler/rustc_trait_selection/src/traits/fulfill.rs:540:                                    .eq(DefineOpaqueTypes::No, a.args, b.args)

and change it to DefineOpaqueTypes::Yes 🙃

[chansuke]

@rustbot claim