[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/FrontendTool/FrontendTool.cpp

@@ -1557,6 +1557,11 @@ static bool validateTBDIfNeeded(const CompilerInvocation &Invocation,
       return false;
     }
 
+    // Embedded Swift does not support TBD.
+    if (Invocation.getLangOptions().hasFeature(Feature::Embedded)) {
+      return false;
+    }
+
     // Cross-module optimization does not support TBD.
     if (Invocation.getSILOptions().CMOMode == CrossModuleOptimizationMode::Aggressive) {
       return false;