SIL: Handle incomplete conformances in TypeConverter::getLoweredCBridgedType()

Currently Sema completes all _ObjectiveCBridgeable conformances it thinks
are needed in SILGen and runtime dynamic casts, but that's about to change.

Make sure SIL passes a LazyResolver down so that if a conformance doesn't
have a type witness for _ObjectiveCType yet, one can be resolved.

Note that in practice, ClangImporter-synthesized conformances use a special
LazyMemberLoader implementation to fill in type witnesses without any
intervention from the type checker. However, that might go away now that
we have a long-lived type checker instance. Furthermore, various tests use
-enable-source-import and start to fail once Sema no longer completes
_ObjectiveCBridgeable conformances.

Author: slavapestov

lib/SIL/Bridging.cpp

@@ -224,7 +224,7 @@ Type TypeConverter::getLoweredCBridgedType(AbstractionPattern pattern,
       ProtocolConformanceRef::getTypeWitnessByName(
         t, ProtocolConformanceRef(conformance),
         M.getASTContext().Id_ObjectiveCType,
-        nullptr);
+        M.getASTContext().getLazyResolver());
     assert(bridgedTy && "Missing _ObjectiveCType witness?");
     if (purpose == BridgedTypePurpose::ForResult && clangTy)
       bridgedTy = OptionalType::get(bridgedTy);