Use SwiftDeclarationAttributeKind instead of string constants

[marcelofabri]

SwiftDeclarationAttributeKind was added in the latest release of SourceKitten 🎉

[SwiftLintBot]

	2 Warnings
⚠️	Please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.
⚠️	This PR may need tests.

	12 Messages
📖	Linting Aerial with this PR took 0.39s vs 0.36s on master (8% slower)
📖	Linting Alamofire with this PR took 3.38s vs 2.91s on master (16% slower)
📖	Linting Firefox with this PR took 13.19s vs 10.68s on master (23% slower)
📖	Linting Kickstarter with this PR took 18.76s vs 15.4s on master (21% slower)
📖	Linting Moya with this PR took 2.23s vs 1.77s on master (25% slower)
📖	Linting Nimble with this PR took 1.62s vs 1.43s on master (13% slower)
📖	Linting Quick with this PR took 0.51s vs 0.48s on master (6% slower)
📖	Linting Realm with this PR took 3.69s vs 2.96s on master (24% slower)
📖	Linting SourceKitten with this PR took 1.04s vs 0.9s on master (15% slower)
📖	Linting Sourcery with this PR took 4.76s vs 4.13s on master (15% slower)
📖	Linting Swift with this PR took 28.42s vs 24.11s on master (17% slower)
📖	Linting WordPress with this PR took 14.39s vs 13.47s on master (6% slower)

Here's an example of your CHANGELOG entry:

* Use SwiftDeclarationAttributeKind instead of string constants.  
  [marcelofabri](https://github.com/marcelofabri)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

Generated by 🚫 Danger

[jpsim]

Now that SourceKitten 0.20.0 is out, can we update this & merge?

[marcelofabri]

jpsim/SourceKitten#512 needs to be released as well

[jpsim]

OK, will do.

[jpsim]

SourceKitten 0.21.0: https://github.com/jpsim/SourceKitten/releases/tag/0.21.0

[codecov-io]

Codecov Report

Merging #2152 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2152      +/-   ##
==========================================
- Coverage   91.84%   91.84%   -0.01%     
==========================================
  Files         269      269              
  Lines       13192    13190       -2     
==========================================
- Hits        12116    12114       -2     
  Misses       1076     1076

Impacted Files	Coverage Δ
...ftLintFramework/Rules/ValidIBInspectableRule.swift	98.63% <100%> (-0.02%)	⬇️
.../SwiftLintFramework/Rules/IdentifierNameRule.swift	98.73% <100%> (ø)	⬆️
...mework/Rules/QuickDiscouragedPendingTestRule.swift	100% <100%> (ø)	⬆️
...ntFramework/Rules/FunctionParameterCountRule.swift	98.43% <100%> (ø)	⬆️
...e/SwiftLintFramework/Rules/PrivateActionRule.swift	100% <100%> (ø)	⬆️
...ce/SwiftLintFramework/Rules/WeakDelegateRule.swift	98.18% <100%> (ø)	⬆️
...SwiftLintFramework/Rules/ProhibitedSuperRule.swift	100% <100%> (ø)	⬆️
...intFramework/Extensions/Dictionary+SwiftLint.swift	87.83% <100%> (+0.16%)	⬆️
...ules/DiscardedNotificationCenterObserverRule.swift	100% <100%> (ø)	⬆️
...LintFramework/Rules/QuickDiscouragedCallRule.swift	100% <100%> (ø)	⬆️
... and 10 more

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