[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/ModuleFile.h

@@ -621,6 +621,11 @@ public:
     return Core->Bits.HasHermeticSealAtLink;
   }
 
+  /// Whether this module was built using embedded Swift.
+  bool isEmbeddedSwiftModule() const {
+    return Core->Bits.IsEmbeddedSwiftModule;
+  }
+
   /// Whether this module was built with C++ interoperability enabled.
   bool hasCxxInteroperability() const {
     return Core->Bits.HasCxxInteroperability;