In a mixed Objective-C / Swift module, we have a Clang module overlay that’s a Source file, not a serialized AST as is currently assumed. That assumption caused a crash when retrieving the symbol graph as part of a cursor info request to SourceKit, which was invoked on a method defined in the Objective-C part of the module.
To fix the crash, recursively use the same logic that already exists to serialize a module to also serialize the clang overlay module since that function alreayd correctly handles the distinction between source files and serialized ASTs.
Resolves rdar://76951147
