swift234

Delay loading the witnesses of a protocol conformance.

Apart from being general compile-time goodness, this helps break a
circularity issue involving serialization cross-references and the
Clang importer.

The test is being added to validation-tests because it relies on
several levels of non-laziness in the compiler, all of which we'd
like to fix. It's making sure we don't regress here, but it isn't
actually verifying this change in particular.

rdar://problem/22364953

Swift SVN r31455