It rebuilds `_allDeclarationsByType` incrementally.
By applying this, the duration of linting Carthage is reduced from:
```
swiftlint lint --config ~/.swiftlint-test.yml 23.05s user 1.85s system 82% cpu 30.338 total
```
to:
```
swiftlint lint --config ~/.swiftlint-test.yml 20.09s user 1.34s system 84% cpu 25.329 total
```
This depends on https://github.com/jpsim/SourceKitten/pull/135
By applying this, the duration of linting Carthage is reduced from:
```
swiftlint lint 27.85s user 1.98s system 77% cpu 38.538 total
```
to:
```
swiftlint lint 21.15s user 1.26s system 80% cpu 27.743 total
```