Frontend: add a frontend flag to generate empty ABI descriptors to workaround deserialization issues

ABI descriptors should always be emitted as sidecars for library-evolution-enabled modules.
However, generating these files requires traversing the entire module (like indexing), which may
hit additional deserialization issues. To unblock builds, this patch introduces a flag to skip
the traversing logic so that we emit an empty ABI descriptor file. The empty file serves as
a placeholder so that build system doesn't need to know the details.

lib/Frontend/Frontend.cpp

@@ -158,7 +158,8 @@ SerializationOptions CompilerInvocation::computeSerializationOptions(
       getIRGenOptions().PublicLinkLibraries;
   serializationOpts.SDKName = getLangOptions().SDKName;
   serializationOpts.ABIDescriptorPath = outs.ABIDescriptorOutputPath.c_str();
-  
+  serializationOpts.emptyABIDescriptor = opts.emptyABIDescriptor;
+
   if (!getIRGenOptions().ForceLoadSymbolName.empty())
     serializationOpts.AutolinkForceLoad = true;