[tool] Include Swift in warning-as-error modes - #13284

I don't know why these analyzer issues are being reported one-at-a-time in CI on #13284 instead of all at once, but #13332 was only one of a few analyzer issues:
```
darwin/Tests/CameraSessionPresetsTests.swift:28:9: error: variable 'currentFormat' was never mutated; consider changing to 'let' constant
    var currentFormat: CaptureDeviceFormat = captureFormatMock
    ~~~ ^
    let
darwin/Tests/CameraInitRaceConditionsTests.swift:62:24: warning: immutable value '_captureSessionQueue' was never used; consider replacing with '_' or removing it [#NoUsage]
60 | 
61 |   func testFlutterChannelInitializedWhenStartingImageStream() {
62 |     let (cameraPlugin, _captureSessionQueue) = createCameraPlugin()
darwin/Tests/AvailableCamerasTests.swift:49:11: warning: variable 'requiredTypes' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
 47 |       telephotoCamera.position = .back
 48 | 
 49 |       var requiredTypes: [AVCaptureDevice.DeviceType] = [
darwin/Tests/AvailableCamerasTests.swift:52:11: warning: variable 'cameras' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
 50 |         .builtInWideAngleCamera, .builtInTelephotoCamera, .builtInUltraWideCamera,
 51 |       ]
 52 |       var cameras = [wideAngleCamera, frontFacingCamera, telephotoCamera, ultraWideCamera]
darwin/Tests/AvailableCamerasTests.swift:86:11: warning: variable 'requiredTypes' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
 84 |       frontFacingCamera.position = .front
 85 | 
 86 |       var requiredTypes: [AVCaptureDevice.DeviceType] = [
darwin/Tests/AvailableCamerasTests.swift:118:11: warning: variable 'requiredTypes' was never mutated; consider changing to 'let' constant [#VariableNeverMutated]
116 |       unspecifiedCamera.position = .unspecified
117 | 
118 |       var requiredTypes: [AVCaptureDevice.DeviceType] = [
darwin/Tests/CameraOrientationTests.swift:120:14: warning: weak variable 'weakDevice' was never mutated; consider changing to 'let' constant [#VariableNeverMutated::WeakMutability]
118 |     let captureSessionQueue = DispatchQueue(label: "capture_session_queue")
119 |     weak var weakPlugin: CameraPlugin?
120 |     weak var weakDevice = mockDevice
darwin/Tests/CameraOrientationTests.swift:157:14: warning: weak variable 'weakEventAPI' was never mutated; consider changing to 'let' constant [#VariableNeverMutated::WeakMutability]
155 |     }
156 | 
157 |     weak var weakEventAPI = mockEventAPI

```

This wasn't showing up in CI due to flutter/flutter#194517.

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