[C-API] Fix a bug in the returning of the build() result, which needs to be live to the caller.
[C-API] Fix a bug in the returning of the build() result, which needs to be live to the caller.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild