[xcodegen] Allow output to be missing in compile_commands.json

This may not always be present for CI builds.

Author: hamishknight

utils/swift-xcodegen/Sources/SwiftXcodeGen/BuildArgs/ClangBuildArgsProvider.swift

@@ -34,16 +34,17 @@ struct ClangBuildArgsProvider {
     // Gather the candidates for each file to get build arguments for. We may
     // have multiple outputs, in which case, pick the first one that exists.
     var commandsToAdd: [RelativePath:
-                          (output: AbsolutePath, args: [Command.Argument])] = [:]
+                          (output: AbsolutePath?, args: [Command.Argument])] = [:]
     for command in parsed {
       guard command.command.executable.knownCommand == .clang,
             let relFilePath = command.file.removingPrefix(repoPath)
       else {
         continue
       }
-      let output = command.directory.appending(command.output)
+      let output = command.output.map { command.directory.appending($0) }
       if let existing = commandsToAdd[relFilePath],
-         existing.output.exists || !output.exists {
+         let existingOutput = existing.output,
+          output == nil || existingOutput.exists || !output!.exists {
         continue
       }
       commandsToAdd[relFilePath] = (output, command.command.args)

utils/swift-xcodegen/Sources/SwiftXcodeGen/Command/CompileCommands.swift

@@ -27,7 +27,7 @@ extension CompileCommands {
   struct Element: Decodable {
     var directory: AbsolutePath
     var file: AbsolutePath
-    var output: RelativePath
+    var output: RelativePath?
     var command: Command
   }
 }

validation-test/BuildSystem/swift-xcodegen.test

@@ -8,8 +8,6 @@
 # REQUIRES: standalone_build
 # REQUIRES: target-same-as-host
 
-# REQUIRES: issue_77407
-
 # First copy swift-xcodegen to the temporary location
 # so we don't touch the user's build, and make sure
 # we're doing a clean build.