To build this:
$ swiftc -emit-object XCBBuildServiceFix.swift -O
$ clang XCBBuildServiceFix.o XCBBuildServiceFix.s -shared -o libXCBBuildServiceFix.dylib -O -L /usr/lib/swift/ -lswiftCore -F /Applications/Xcode.app/Contents/SharedFrameworks/XCBuild.framework/Versions/Current/PlugIns/XCBBuildService.bundle/Contents/Frameworks/ -framework XCBUtil -framework XCBProtocol
Then inject the result into XCBBuildService, which launches from /Applications/Xcode.app/Contents/SharedFrameworks/XCBuild.framework/Versions/Current/PlugIns/XCBBuildService.bundle/Contents/MacOS/XCBBuildService, using your injection tool of choice.