[Serialization] Support (de-)serialization of SubstitutionMaps.

Allow substitution maps to be serialized directly (via an ID), writing out
the replacement types and conformances as appropriate. This is a more
efficient form of serialization than the current SubstitutionList approach,
because it maintains uniqueness of substitution maps within a module file,
and is a step toward eliminating SubstitutionList entirely.

lib/Serialization/ModuleFile.cpp

@@ -852,6 +852,10 @@ bool ModuleFile::readIndexBlock(llvm::BitstreamCursor &cursor) {
         assert(blobData.empty());
         GenericEnvironments.assign(scratch.begin(), scratch.end());
         break;
+      case index_block::SUBSTITUTION_MAP_OFFSETS:
+        assert(blobData.empty());
+        SubstitutionMaps.assign(scratch.begin(), scratch.end());
+        break;
       case index_block::NORMAL_CONFORMANCE_OFFSETS:
         assert(blobData.empty());
         NormalConformances.assign(scratch.begin(), scratch.end());