Include argsfile field in [linker.command] JSON

Summary:
Previously the argsfile was included in the command field of the JSON output,
but it is easier to write tests against the argsfile if it has it's own field in the produced JSON.

This diff makes that change.

Reviewed By: rmaz

Differential Revision: D59840769

fbshipit-source-id: 2986995a3f69974fcd6bd219757a7f4f2844906e

prelude/linking/link_info.bzl

@@ -983,6 +983,7 @@ def make_link_command_debug_output_json_info(ctx: AnalysisContext, debug_outputs
             associated_artifacts.extend([debug_output.dist_thin_lto_codegen_argsfile, debug_output.dist_thin_lto_index_argsfile])
         else:
             json_info.append({
+                "argsfile": debug_output.argsfile,
                 "command": debug_output.command,
                 "filename": debug_output.filename,
             })