[Sema] Avoid computing raw values for enum cases in swiftinterface files
Raw values of enum cases from another module are not specified in the declaration of the enum unless that enum is `@objc`. This meant that `EnumRawValuesRequest` was computing potentially incorrect raw values when the enum declaration supported it and was emitting incorrect diagnostics for other enum decls.
Resolves SR-14355 and rdar://75451691