Swift compatibility with Swift 4.2, 5.0 and 5.1

It was reported on Stack Overflow (https://stackoverflow.com/questions/56841613/how-to-import-thrift-0-12-into-xcode-to-support-swift-4-2) that Thrift doesn't work on Swift 4.2. I was able to confirm that issue, so I made a pull request to solve the problem: https://github.com/apache/thrift/pull/1827

The pull request keeps compatibility with Swift 3 (which is deprecated and won't be usable at all on macOS Catalina), and adds support to Swift 4.0, 4.2, 5.0 and 5.1.

Related: THRIFT-4547.