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.
It is still not clear to me when we access global variables from other
modules directly, versus using accessors; it seems to be controlled
by the -sil-serialize-all flag, rather than any language feature.
Until/if we add a @_fixed_layout equivalent for globals, I can't really
test direct access of globals from other modules; when we figure out
the story here I'll be able to add more tests and also tighten up
some isResilient() checks in the global code, but what's in there now
seems to work.
