Fix manual_let_else FN when diverges on simple enum variant

[profetia]

Closes #14598

Implemented checks for simple enum variants when checking if match arms are disjointed.

changelog: [manual_let_else] fix FN when diverges on simple enum variant

[rustbot]

r? @dswij

rustbot has assigned @dswij.
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

[profetia]

Hmm.. Why isn't this PR merged automatically?

[samueltardieu]

The PR are never merged automatically in this repository, the assigned reviewer has to press the "Merge when ready" button.

[dswij]

Whoops yeah, I might've forgotten to click the merge button. 😅

[nyurik]

thank you!