Update `isSDKTooOld` to be compatible with older compilers. 5.3 and ealier compilers may hit: ``` swift-driver/Sources/SwiftDriver/Driver/Driver.swift:2044:15: error: definition conflicts with previous value     guard let sdkInfo = sdkInfo else { ```
Update `isSDKTooOld` to be compatible with older compilers.
5.3 and ealier compilers may hit:
```
swift-driver/Sources/SwiftDriver/Driver/Driver.swift:2044:15: error: definition conflicts with previous value
    guard let sdkInfo = sdkInfo else {
```

apple/swift-driver,apple/swift-driver,apple/swift-driver,apple/swift-driver