SwiftLint2

Optimize `File.allDeclarationsByType`

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
```