For two reasons:
* We also like to check for assert failures in release builds. Although this could be achieved with `precondition`, it's easy to forget about it and use `assert` instead.
* We need to see the error message in crashlogs of release builds. This is even not the case for `precondition`.

Also, re-export the "Basic" module in "SIL" so that the new assert implementation is also available in the Optimizer module (where all files import SIL).

…ion`

It's better to use the new assert implementation (defined in the "Basic" module) than Swift's `precondition`