[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

include/swift/Serialization/SerializationOptions.h

@@ -155,6 +155,7 @@ namespace swift {
     bool DisableCrossModuleIncrementalInfo = false;
     bool StaticLibrary = false;
     bool HermeticSealAtLink = false;
+    bool EmbeddedSwiftModule = false;
     bool IsOSSA = false;
     bool SerializeExternalDeclsOnly = false;
   };