[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/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 803; // removed initializes and accesses attributes
+const uint16_t SWIFTMODULE_VERSION_MINOR = 804; // embedded swift modules
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -885,6 +885,7 @@ namespace options_block {
     IS_SIB,
     IS_STATIC_LIBRARY,
     HAS_HERMETIC_SEAL_AT_LINK,
+    IS_EMBEDDED_SWIFT_MODULE,
     IS_TESTABLE,
     RESILIENCE_STRATEGY,
     ARE_PRIVATE_IMPORTS_ENABLED,
@@ -928,6 +929,10 @@ namespace options_block {
     HAS_HERMETIC_SEAL_AT_LINK
   >;
 
+  using IsEmbeddedSwiftModuleLayout = BCRecordLayout<
+    IS_EMBEDDED_SWIFT_MODULE
+  >;
+
   using IsTestableLayout = BCRecordLayout<
     IS_TESTABLE
   >;