[APIDigester] Allow outputting module diff diagnostics as JSON for other tools to consume

[owenv]

The motivation behind this change is to begin experimenting with automatic semantic versioning suggestions for Swift packages. By serializing the kind of API change in addition to the diagnostic text, it gives tools which use the api digester a little more flexibility in how they interpret the output of a -diagnose-sdk action. The new behavior is gated behind the --json flag.

Example Output:

{
  "version": {
    "major": 0,
    "minor": 1
  },
  "diagnostics": [
    {
      "identifier": "declTypeChange",
      "text": "cake: Constructor S1.init(_:) has parameter 0 type change from Swift.Int to Swift.Double"
    },
    {
      "identifier": "funcSelfAccessChange",
      "text": "cake: Func S1.foo1() has self access kind changing from NonMutating to Mutating"
    },
...

[owenv]

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