[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/SILOptimizer/PassManager/PassPipeline.cpp

@@ -1004,6 +1004,9 @@ SILPassPipelinePlan::getOnonePassPipeline(const SILOptions &Options) {
   // inlinable functions from imported ones.
   P.addOnonePrespecializations();
 
+  // For embedded Swift: CMO is used to serialize libraries.
+  P.addCrossModuleOptimization();
+
   // First serialize the SIL if we are asked to.
   P.startPipeline("Serialization");
   P.addSerializeSILPass();