Use ExtensionDecl::getExtendedNominal() to wire up extensions to their
nominal types early in type checking (the bindExtensions()) operation,
rather than going through type validation to do so.
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.