Xcode 7.3 Syntax Highlighting and Code Completion issues with Swift

I have the same problem. But finally solved it.
I make two change, not sure which is the key point but you can try them all.

1. delete the module cache

Within the same folder as your project’s Derived Data is a Module
Cache. When Code Completion stopped working, deleting this fixed it.

Close Xcode and delete the
~/Library/Developer/Xcode/DerivedData/ModuleCache directory.

2. change the Enable Modules value

Go to the Build Settings of your target, then search Enable
Modules

If it’s Yes, change it to No, and you may get some build
error, just change it back to Yes.

After two steps above you should Clean(Shift+Command+K) your project.

For now you may fixed the problem.