[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/SerializedModuleLoader.cpp

@@ -826,6 +826,7 @@ LoadedFile *SerializedModuleLoaderBase::loadAST(
     fileUnit = new (Ctx) SerializedASTFile(M, *loadedModuleFile);
     M.setStaticLibrary(loadedModuleFile->isStaticLibrary());
     M.setHasHermeticSealAtLink(loadedModuleFile->hasHermeticSealAtLink());
+    M.setIsEmbeddedSwiftModule(loadedModuleFile->isEmbeddedSwiftModule());
     if (loadedModuleFile->isTestable())
       M.setTestingEnabled();
     if (loadedModuleFile->arePrivateImportsEnabled())
@@ -916,6 +917,18 @@ LoadedFile *SerializedModuleLoaderBase::loadAST(
                        diag::need_hermetic_seal_to_import_module, M.getName());
   }
 
+  if (M.isEmbeddedSwiftModule() &&
+      !Ctx.LangOpts.hasFeature(Feature::Embedded)) {
+    Ctx.Diags.diagnose(diagLoc.value_or(SourceLoc()),
+                       diag::cannot_import_embedded_module, M.getName());
+  }
+
+  if (!M.isEmbeddedSwiftModule() &&
+      Ctx.LangOpts.hasFeature(Feature::Embedded)) {
+    Ctx.Diags.diagnose(diagLoc.value_or(SourceLoc()),
+                       diag::cannot_import_non_embedded_module, M.getName());
+  }
+
   // Non-resilient modules built with C++ interoperability enabled
   // are typically incompatible with clients that do not enable
   // C++ interoperability.