Swift4: Fix issue 6746 - problem with arrays of primitives

[ehyche]

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Description of the PR

This change fixes this issue for swift4: #6746

I will submit a separate PR for swift3.

This should only be merged AFTER this PR is merged: #7071

Previously, when we had these three definitions:

    "ModelStringArray": {
      "type": "array",
      "description": "This defines an array of strings.",
      "items": {
        "type": "string"
      }
    },
    "ModelDoubleArray": {
      "type": "array",
      "description": "This defines an array of doubles.",
      "items": {
        "type": "number",
        "format": "double"
      }
    },
    "ModelErrorInfoArray": {
      "type": "array",
      "description": "This defines an array of ErrorInfo objects.",
      "items": {
        "$ref": "#/definitions/ErrorInfo"
      }
    },

then swift4 would generate the following:

public typealias ModelStringArray = []
public typealias ModelDoubleArray = []
public typealias ModelErrorInfoArray = [ErrorInfo]

and clearly only the generated code for ModelErrorInfoArray is correct. The problem was that our template had:

{{#isArrayModel}}
public typealias {{classname}} = [{{arrayModelType}}]
{{/isArrayModel}}

and arrayModelType is not present for primitives like String, Double, etc. However, the "parent" is always present and is correct. So I changed the template to:

{{#isArrayModel}}
public typealias {{classname}} = {{parent}}
{{/isArrayModel}}

[jaz-ah]

@wing328 +1 thx @ehyche