Write output-file-map.json atomically

There are two subtle changes in behavior here:
- The old variant of `writeFileContents` went through a few abstraction layers in `FileSystem+Extensions.swift` that ended up calling `writeFileContents(_:body:)`, which also created the directories to the file. think we can assume that the parent directory already exists here.
- This call will fail if the file system doesn’t support atomic operations. I’m not sure if we need to support file systems that don’t allow atomic operations here.

rdar://124727242

Author: ahoppen

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -779,7 +779,7 @@ package final class SwiftTargetBuildDescription {
 
         content += "}\n"
 
-        try self.fileSystem.writeFileContents(path, string: content)
+        try self.fileSystem.writeFileContents(path, bytes: .init(encodingAsUTF8: content), atomically: true)
         return path
     }