[embedded] Add basics of module serialization, importing and validation in embedded Swift.

- Add a flag to the serialized module (IsEmbeddedSwiftModule)
- Check on import that the mode matches (don't allow importing non-embedded module in embedded mode and vice versa)
- Drop TBD support, it's not expected to work in embedded Swift for now
- Drop auto-linking backdeploy libraries, it's not expected to backdeploy embedded Swift for now
- Drop prespecializations, not expected to work in embedded Swift for now
- Use CMO to serialize everything when emitting an embedded Swift module
- Change SILLinker to deserialize/import everything when importing an embedded Swift module
- Add an IR test for importing modules
- Add a deserialization validation test

lib/Serialization/Serialization.cpp

@@ -1038,6 +1038,11 @@ void Serializer::writeHeader() {
         HasHermeticSealAtLink.emit(ScratchRecord);
       }
 
+      if (Options.EmbeddedSwiftModule) {
+        options_block::IsEmbeddedSwiftModuleLayout IsEmbeddedSwiftModule(Out);
+        IsEmbeddedSwiftModule.emit(ScratchRecord);
+      }
+
       if (M->isTestingEnabled()) {
         options_block::IsTestableLayout IsTestable(Out);
         IsTestable.emit(ScratchRecord);