add a "references" relationship kind and referenceLocation mixin

[QuietMisdreavus]

Bug/issue #, if applicable: rdar://108470705

Summary

This PR adds a new relationship kind, references, that describes a relationship where one symbol references another one in its implementation. It also introduces a referenceLocation mixin for relationships that describes the source range where the reference occurs.

The idea behind adding this information to a symbol graph would be to allow for more semantic analyses to be made on projects, for example to find unused symbols within a project. Including the source range also allows for a sort of "offline language server", by providing a data source for a "jump to definition" and "find references" functionality.

The JSON for these relationships would look like this:

{
  "kind": "references",
  "source": "source",
  "target": "target",
  "referenceLocation": {
    "uri": "file://file.swift",
    "range": {
      "start": {
        "line": 14,
        "character": 0
      },
      "end": {
        "line": 14,
        "character": 6
      }
    }
  }
}

Dependencies

None

Testing

This PR is being used as a baseline specification for future implementations, so demonstration symbol graphs need to be synthesized from whole cloth and cannot be tested in a straightforward manner.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[QuietMisdreavus]

@swift-ci Please test

[daniel-grumberg]

LGTM