Auto merge of rust-lang#105094 - Swatinem:generator-not-future, r=compiler-errors

Make sure async constructs do not `impl Generator`

Async lowering turns async functions and blocks into generators internally.
Though these special kinds of generators should not `impl Generator` themselves.
The other way around, normal generators should not `impl Future`.

This was discovered in rust-lang#105082 (comment) and is a regression from rust-lang#104321.

r? `@compiler-errors`

Author: bors