[GSB] Hack: self-derived sources are showing up very late; ignore them.

There's a more pervasive issue here that needs further study;
essentially, we can end up with self-derived sources getting
reinjected later, and will either need to avoid that reinjection or
need to expand what this localized hack does, filtering out
self-derived constraints when we enumerate requirements.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -4211,6 +4211,15 @@ static void computeDerivedSameTypeComponents(
     assert(componentOf.count(concrete.archetype) > 0);
     auto &component = components[componentOf[concrete.archetype]];
 
+    // FIXME: Skip self-derived sources. This means our attempts to "stage"
+    // construction of self-derived sources really don't work, because we
+    // discover more information later, so we need a more on-line or
+    // iterative approach.
+    bool derivedViaConcrete;
+    if (concrete.source->isSelfDerivedSource(concrete.archetype,
+                                             derivedViaConcrete))
+      continue;
+
     // If it has a better source than we'd seen before for this component,
     // keep it.
     auto &bestConcreteTypeSource = component.concreteTypeSource;