Do not look for disallowed methods inside desugared code

[samueltardieu]

changelog: [disallowed_methods]: do not check compiler-generated code

Fixes #16185

[rustbot]

r? @blyxyas

rustbot has assigned @blyxyas.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[samueltardieu]

Repushing with a better commit message

[samueltardieu]

@blyxyas is currently out of reviewer rotation.
r? clippy