[compiler] Quick fix of --code-comments

Enabling background code assembly broke --code-comments. This CL "fixes"
it for now by simply skipping the printing of inlining stack info.

R=bmeurer@chromium.org

Bug: v8:6048
Change-Id: I8b9ea629c7c302198484510a7234014bfbfd890b
Reviewed-on: https://chromium-review.googlesource.com/569163Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46617}

src/compiler/code-generator.cc

@@ -508,7 +508,9 @@ void CodeGenerator::AssembleSourcePosition(SourcePosition source_position) {
     if (FLAG_trace_turbo) {
       buffer << source_position;
     } else {
-      buffer << source_position.InliningStack(info);
+      buffer << source_position;
+      // TODO(neis): Figure out if/how to print InliningStack(info) from a
+      // background thread.
     }
     buffer << " --";
     tasm()->RecordComment(StrDup(buffer.str().c_str()));