[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/Frontend/Frontend.cpp

@@ -223,6 +223,9 @@ SerializationOptions CompilerInvocation::computeSerializationOptions(
 
   serializationOpts.HermeticSealAtLink = opts.HermeticSealAtLink;
 
+  serializationOpts.EmbeddedSwiftModule =
+      LangOpts.hasFeature(Feature::Embedded);
+
   serializationOpts.IsOSSA = getSILOptions().EnableOSSAModules;
 
   serializationOpts.SerializeExternalDeclsOnly =