[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/SIL/IR/SILModule.cpp

@@ -898,6 +898,10 @@ void SILModule::performOnceForPrespecializedImportedExtensions(
   if (prespecializedFunctionDeclsImported)
     return;
 
+  // No prespecitalizations in embedded Swift
+  if (getASTContext().LangOpts.hasFeature(Feature::Embedded))
+    return;
+
   SmallVector<ModuleDecl *, 8> importedModules;
   // Add the Swift module.
   if (!isStdlibModule()) {