Fix identifier_name rule when using Swift 4.2

[marcelofabri]

Fixes #2231

Continued from #2232 // cc @Coder-256

[SwiftLintBot]

	12 Messages
📖	Linting Aerial with this PR took 0.38s vs 0.35s on master (8% slower)
📖	Linting Alamofire with this PR took 3.12s vs 3.13s on master (0% faster)
📖	Linting Firefox with this PR took 11.81s vs 11.93s on master (1% faster)
📖	Linting Kickstarter with this PR took 17.24s vs 17.11s on master (0% slower)
📖	Linting Moya with this PR took 1.89s vs 1.83s on master (3% slower)
📖	Linting Nimble with this PR took 1.59s vs 1.52s on master (4% slower)
📖	Linting Quick with this PR took 0.5s vs 0.48s on master (4% slower)
📖	Linting Realm with this PR took 3.15s vs 3.08s on master (2% slower)
📖	Linting SourceKitten with this PR took 1.01s vs 0.97s on master (4% slower)
📖	Linting Sourcery with this PR took 4.52s vs 4.35s on master (3% slower)
📖	Linting Swift with this PR took 28.1s vs 26.85s on master (4% slower)
📖	Linting WordPress with this PR took 15.09s vs 14.69s on master (2% slower)

Generated by 🚫 Danger

[codecov-io]

Codecov Report

Merging #2255 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #2255      +/-   ##
==========================================
- Coverage   92.14%   92.13%   -0.01%     
==========================================
  Files         286      286              
  Lines       14515    14522       +7     
==========================================
+ Hits        13375    13380       +5     
- Misses       1140     1142       +2

Impacted Files	Coverage Δ
.../SwiftLintFramework/Rules/IdentifierNameRule.swift	96.51% <75%> (-2.23%)	⬇️

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