swift

bdb25f4f - [AST] SE-0487: Promote `NonexhaustiveAttribute` to a language feature

[AST] SE-0487: Promote `NonexhaustiveAttribute` to a language feature The proposal [has been accepted](https://forums.swift.org/t/accepted-se-0487-nonexhaustive-enums/81508) which makes the feature experimental no longer. (cherry picked from commit a7ecd309d4577b7974a2c912db3f3b23f83a61be)