Ignore TODOs in newly added analysis_server benchmark

Since the benchmark has its own `analysis_options.yaml` file, the
files inside it are analyzed with default analysis options, meaning
that `TODO` comments inside the benchmark are now showing up in the
IDE, which is distracting when making edits to the analysis server.

Change-Id: I42b02da06d6754a26157bcf0c07b8d5c4b2ac31d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/414240
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: stereotype441

pkg/analysis_server/tool/benchmark_tools/big_chain_benchmark/files/analysis_options.yaml

@@ -0,0 +1,3 @@
+analyzer:
+  errors:
+    todo: ignore