Add `free()` workaround that causes error when built with some compiler versions. Otherwise some building compilers will see errors: ``` error: value of optional type 'UnsafeMutablePointer<CChar>?' (aka 'Optional<UnsafeMutablePointer<Int8>>') must be unwrapped to a value of type 'UnsafeMutablePointer<CChar>' (aka 'UnsafeMutablePointer<Int8>')   for ptr in cstrings { free(ptr) } ```
Add `free()` workaround that causes error when built with some compiler versions.
Otherwise some building compilers will see errors:
```
error: value of optional type 'UnsafeMutablePointer<CChar>?' (aka 'Optional<UnsafeMutablePointer<Int8>>') must be unwrapped to a value of type 'UnsafeMutablePointer<CChar>' (aka 'UnsafeMutablePointer<Int8>')
  for ptr in cstrings { free(ptr) }
```

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