swift

b493b56e - Sema: Remove `OpaqueUnderlyingTypeChecker::AvailabilityContext`.

Sema: Remove `OpaqueUnderlyingTypeChecker::AvailabilityContext`. `AvailabilityContext` is now a fundamental type used throughout the compiler, so it's confusing for `OpaqueUnderlyingTypeChecker` to repurpose this name to represent an `IfStmt`. NFC.