[6.2][AST/Sema] SE-0487: Implement `@nonexhaustive` attribute and new enum exhaustivity checking rule#82922

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xedin:nonexhaustive-enums-6.2

Commits on Jul 8, 2025

[AST/Sema] Add `@extensible` attribute on `enum` declarations
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[Frontend/Serialization] Remove `ExtensibleEnums` experimental flag
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Commits on Jul 9, 2025

[Frontend] Add `ExtensibleAttribute` to guard use of `@extensible` attribute
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[AST] Prevent use of `@extensible` without `ExtensibleAttribute` feature
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[AST/Sema] Add `@preEnumExtensibility` attribute to downgrade warnings for extensible enums
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[AST/Sema] SE-0487: Rename `@extensible` into `@nonexhaustive`
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[AST/Sema] SE-0487: Expand `@nonexhaustive` attribute to support `warn` argument
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[AST/Sema] SE-0487: Remove `@preEnumExtensibility` attribute
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