[Serialization] Wire up the generic parameter of a protocol early.

When we deserialize a protocol declaration, we had a bunch of
intervening code before it would deserialize and wire up its generic
parameter (Self), including configuring the generic environment. If
any of that code referenced an AssociatedTypeDecl of that protocol,
the deserialization of the AssociatedTypeDecl could end up computing
an erroneous type.

This might be the source of the nondeterministic deserialization
failure that seems to afflict AssociatedTypeDecls in
rdar://problem/30382791.

Author: DougGregor

lib/Serialization/Deserialization.cpp

@@ -2576,6 +2576,10 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
     declOrOffset = assocType;
 
     assocType->computeType();
+
+    assert(!assocType->getDeclaredInterfaceType()->hasError() &&
+           "erroneous associated type");
+
     Accessibility parentAccess = cast<ProtocolDecl>(DC)->getFormalAccess();
     assocType->setAccessibility(std::max(parentAccess,Accessibility::Internal));
     if (isImplicit)
@@ -3031,8 +3035,6 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
                                           getIdentifier(nameID), None);
     declOrOffset = proto;
 
-    configureGenericEnvironment(proto, genericEnvID);
-
     proto->setRequiresClass(isClassBounded);
 
     if (auto accessLevel = getActualAccessibility(rawAccessLevel)) {
@@ -3042,6 +3044,10 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
       return nullptr;
     }
 
+    auto genericParams = maybeReadGenericParams(DC);
+    assert(genericParams && "protocol with no generic parameters?");
+    proto->setGenericParams(genericParams);
+
     auto protocols = ctx.Allocate<ProtocolDecl *>(numProtocols);
     for_each(protocols, rawProtocolAndInheritedIDs.slice(0, numProtocols),
              [this](ProtocolDecl *&p, uint64_t rawID) {
@@ -3051,9 +3057,7 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
 
     handleInherited(proto, rawProtocolAndInheritedIDs.slice(numProtocols));
 
-    auto genericParams = maybeReadGenericParams(DC);
-    assert(genericParams && "protocol with no generic parameters?");
-    proto->setGenericParams(genericParams);
+    configureGenericEnvironment(proto, genericEnvID);
 
     SmallVector<Requirement, 4> requirements;
     readGenericRequirements(requirements, DeclTypeCursor);