[SourceKit] Add subscript to doc structure (SR-5035)

[marcelofabri]

This adds subscript to the structure reported by SourceKit.

For example, for this file:

class Foo {
    subscript(index: Int) -> Int {
        get {
            return 0
        }
        set {
            print(newValue)
        }
    }
}

We get this structure now:

{
  key.offset: 0,
  key.length: 104,
  key.diagnostic_stage: source.diagnostic.stage.swift.parse,
  key.substructure: [
    {
      key.kind: source.lang.swift.decl.class,
      key.accessibility: source.lang.swift.accessibility.internal,
      key.name: "Foo",
      key.offset: 0,
      key.length: 103,
      key.runtime_name: "_TtC8__main__3Foo",
      key.nameoffset: 6,
      key.namelength: 3,
      key.bodyoffset: 11,
      key.bodylength: 91,
      key.substructure: [
        {
          key.kind: source.lang.swift.decl.function.subscript,
          key.accessibility: source.lang.swift.accessibility.internal,
          key.setter_accessibility: source.lang.swift.accessibility.internal,
          key.name: "subscript(_:)",
          key.offset: 13,
          key.length: 88,
          key.nameoffset: 0,
          key.namelength: 0,
          key.bodyoffset: 43,
          key.bodylength: 57,
          key.substructure: [
            {
              key.kind: source.lang.swift.decl.var.parameter,
              key.name: "index",
              key.offset: 23,
              key.length: 10,
              key.typename: "Int",
              key.nameoffset: 0,
              key.namelength: 0
            }
          ]
        },
        {
          key.kind: source.lang.swift.expr.call,
          key.name: "print",
          key.offset: 79,
          key.length: 15,
          key.nameoffset: 79,
          key.namelength: 5,
          key.bodyoffset: 85,
          key.bodylength: 8
        }
      ]
    }
  ]
}

Resolves SR-5035.

This wouldn't be possible without the help from @johnfairh and @CodaFi. Thanks so much! 🙌

[marcelofabri]

I'm not sure if this is the proper solution or if it's better to:

1. Rename this function
2. Whenever using isVariable also check if Kind == Subscript

[nkcsgexi]

what happens if we don't add this line here?

[marcelofabri]

the syntax kinds of get and set inside subscripts become identifier instead of keyword

[CodaFi]

This accessor (isVariable()) is only being used in one place in the Syntax Model and only for things that have substructure that needs to be walked. Would it be OK to just rename it hasSubstructure() or something?

[nkcsgexi]

I agree with @CodaFi . Renaming it sounds a right approach to me.

[marcelofabri]

Renamed and amended the commit.

[marcelofabri]

cc @nkcsgexi

[nkcsgexi]

@swift-ci please smoke test and merge