swift234

Excise the global TranslationUnit from TypeChecker.

Now that TypeChecker is being used to check all sorts of things (not all
from a single TU), it's not really correct to have a single top-level TU
that gets used everywhere. Instead, we should be using the TU that's
appropriate for whatever's being checked. This is a small correctness win
for order-independent type-checking, but is critical for multi-file
translation units, which is needed for implicit visibility.

This basically involves passing around DeclContexts much more.

Caveat: we aren't smart about, say, filtering extensions based on the
current context, so we're still not 100% correct here.

Swift SVN r9006