[Serialization] Add warning when .swiftsourceinfo is malformed

Output a warning rather than ignoring a malformed .swiftsourceinfo
completely.

Resolves rdar://77350048

lib/Serialization/ModuleFileSharedCore.cpp

@@ -1490,3 +1490,7 @@ ModuleFileSharedCore::ModuleFileSharedCore(
     return;
   }
 }
+
+bool ModuleFileSharedCore::hasSourceInfo() const {
+  return !!DeclUSRsTable;
+}