This is needed if we compile StdlibUnittest with -sil-serialize-all
So far I added the imports only in files which needed them. But this may change, depending on the optimizer (inlining).
Adding them in all files doesn't harm and avoids confusion if someone makes an unrelated change which would result in such a linker error.
Per Dmitri's request and stdlib conventions. Also, remove the reference to
what's now '_value' from the DarwinBoolean tests, and use unsafeBitCast
instead to mimic representation-compatible access from C.
Swift SVN r30066
