Issue Description:

SourceKitService keeps using more and more memory for every swift file that i open in a project.

It seems that the claimed memory is never released till the user restarts Xcode.

Would it be possible to have an upper limit on the amount of memory sourcekit could use and then keep cycling files in FIFO order, and maintain a memory usage of around 1 GB or something?

Example:

• clone this repo (it's a fork of https://github.com/danielgindi/Charts) with code updated to compile for swift4
  https://github.com/asifmohd/Charts/tree/swift4

• open the swift files in the project one by one using Xcode 8.3.2, and notice that SourceKitService keeps using memory around 42 MB for every swift file that you open.

• if you do this for all the files in the project it shoots up all the way to 5.5 GB!

This impacts everyday development in swift for large projects, as we have to restart Xcode every few hours just to reclaim the memory lost to SourceKitService.