Sema: Don't set type of AssociatedTypeDecl until we have an archetype

Otherwise the verifier can crash because hasType() returns true but
getType() gives us a MetatypeType that hits a null pointer in
desugaring.

The computeType() calls appear in a few too many places for my liking;
would be nice to clean this up further or replace everything with
interface types one day.

Fixes <rdar://problem/19606899>.

Swift SVN r30388

lib/Serialization/Deserialization.cpp

@@ -2155,6 +2155,7 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
                                                     defaultDefinitionID);
     declOrOffset = assocType;
 
+    assocType->computeType();
     assocType->setAccessibility(cast<ProtocolDecl>(DC)->getFormalAccess());
     assocType->setSuperclass(getType(superclassID));
     assocType->setArchetype(getType(archetypeID)->castTo<ArchetypeType>());