Allow SwiftCompilerMessages with no outputs

Swift frontend jobs are not required to have outputs. When they don’t, there is no “output” field in the driver’s JSON. SwiftPM previously assumed an “output” field must be present; this commit disabuses it of that notion.

Author: beccadax

Sources/Build/SwiftCompilerOutputParser.swift

@@ -28,14 +28,14 @@ public struct SwiftCompilerMessage {
         public struct BeganInfo {
             public let pid: Int
             public let inputs: [String]
-            public let outputs: [Output]
+            public let outputs: [Output]?
             public let commandExecutable: String
             public let commandArguments: [String]
 
             public init(
                 pid: Int,
                 inputs: [String],
-                outputs: [Output],
+                outputs: [Output]?,
                 commandExecutable: String,
                 commandArguments: [String]
             ) {
@@ -49,7 +49,7 @@ public struct SwiftCompilerMessage {
 
         public struct SkippedInfo {
             public let inputs: [String]
-            public let outputs: [Output]
+            public let outputs: [Output]?
 
             public init(inputs: [String], outputs: [SwiftCompilerMessage.Kind.Output]) {
                 self.inputs = inputs

Tests/BuildTests/SwiftCompilerOutputParserTests.swift

@@ -57,6 +57,17 @@ class SwiftCompilerOutputParserTests: XCTestCase {
               ],
               "pid": 58776
             }
+            250
+            {
+              "kind": "began",
+              "name": "verify-module-interface",
+              "inputs": [
+                "main.swiftinterface"
+              ],
+              "pid": 31337,
+              "command_executable": "swift",
+              "command_arguments" : ["-frontend", "-typecheck-module-from-interface", "main.swiftinterface"]
+            }
             299
             {
               "kind": "began",
@@ -108,6 +119,14 @@ class SwiftCompilerOutputParserTests: XCTestCase {
                     outputs: [.init(
                         type: "object",
                         path: "/var/folders/yc/rgflx8m11p5d71k1ydy0l_pr0000gn/T/test2-77d991.o")]))),
+            SwiftCompilerMessage(
+                name: "verify-module-interface",
+                kind: .began(.init(
+                    pid: 31337,
+                    inputs: ["main.swiftinterface"],
+                    outputs: nil,
+                    commandExecutable: "swift",
+                    commandArguments: ["-frontend", "-typecheck-module-from-interface", "main.swiftinterface"]))),
             SwiftCompilerMessage(
                 name: "link",
                 kind: .began(.init(