Reduce memory usage (#4349)

When linting SwiftLint, this brings memory usage down by around 20%,
going from 372MB to 297MB.

This is achieved by removing the unused `RebuildQueue` and by clearing
cached data associated with files after processing them.

Depends on https://github.com/jpsim/SourceKitten/pull/749.

Source/SwiftLintFramework/Models/Linter.swift

@@ -249,6 +249,9 @@ public struct CollectedLinter {
                 "completely removed in a future release.")
         }
 
+        // Free some memory used for this file's caches. They shouldn't be needed after this point.
+        file.invalidateCache()
+
         return (violations, ruleTimes)
     }