[BuildSystem] Fix use after free.
[BuildSystem] Fix use after free.

 - Found by a newer version of ASAN!

 - <rdar://problem/31561038> Bot Issue for llbuild-asan (test assertion in -[LitTests testllbuild____BuildSystem_Build_archive_llbuild])

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