[Generic environment] Weaken an assertion when forming a substitution map.

It turns out that GenericSignature::getAllDependentTypes() sometimes
includes generic type parameter types that have been made
concrete. Tolerate this for now, because fixing it causes issues
elsewhere.

Author: DougGregor

lib/AST/GenericEnvironment.cpp

@@ -412,7 +412,9 @@ getSubstitutionMap(SubstitutionList subs) const {
       continue;
     }
 
-    assert(contextTy->hasError());
+    // FIXME: getAllDependentTypes() includes generic type parameters that
+    // have been made concrete.
+    assert(contextTy->hasError() || depTy->is<GenericTypeParamType>());
   }
 
   assert(subs.empty() && "did not use all substitutions?!");