This is needed for tests which define internal functions which should not be eliminated.
So far this was not needed because of a hack which prevented whole-module-optimizations for tests.
Swift SVN r22658
In most cases this means adding @public to things that get serialized;
in a few cases it means using a modern public stdlib API instead of
a legacy thing I was trying to keep @internal.
Swift SVN r19350
Entities with shared linkage are allowed to be discarded if they are unused even
in a library context.
Previously we implemented this in the serializer, which introduced
needless complications. Now we leave that responsibility to the optimizer giving
simplicity.
Swift SVN r16150
