[Serialization] Add assertions to track down a SIL deserialization issue.

The issue: we're apparently not keeping the archetypes in an instruction in sync
with the archetypes used in the SIL function's generic signature. This apparently
isn't the problem, but it's a good assertion to have, if a little ad hoc.

(The actual issue is rdar://problem/23892955.)

Author: jrose-apple

lib/Serialization/Deserialization.cpp

@@ -579,6 +579,11 @@ ModuleFile::maybeReadSubstitution(llvm::BitstreamCursor &cursor) {
                                                           replacementID,
                                                           numConformances);
 
+  if (&cursor == &SILCursor) {
+    assert(Types[archetypeID-1].isComplete() &&
+	   "SIL substitutions should always reference existing archetypes");
+  }
+
   auto archetypeTy = getType(archetypeID)->castTo<ArchetypeType>();
   auto replacementTy = getType(replacementID);