Ignore TODOs in analyzer_cli package.

Some time ago, the analyzer team decided to adopt the convention of
using `TODO` comments to document long term issues that should persist
in the codebase, and `FIXME` comments to document short term issues
that need immediate attention.

Accordingly, it makes sense to suppress `TODO` comments from being
surfaced to the IDE "problems" view (since there can be a lot of them,
and they're not immediately actionable).  This makes VSCode's
"problems" view much more usable in "tree" mode.

Previous CLs that have made this change in other packages:
- https://dart-review.googlesource.com/c/sdk/+/295662
- https://dart-review.googlesource.com/c/sdk/+/325121
- https://dart-review.googlesource.com/c/sdk/+/358980
- https://dart-review.googlesource.com/c/sdk/+/410060
- https://dart-review.googlesource.com/c/sdk/+/440546

Change-Id: I2df5fc3ad1da3d2654622e5e276ed3e3a841eb7f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442167
Reviewed-by: Konstantin Shcheglov <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer_cli/analysis_options.yaml

@@ -8,6 +8,9 @@ analyzer:
     # We import heavily from package:analyzer/src.
     implementation_imports: ignore
     non_constant_identifier_names: ignore
+    # Ignoring this for all developers means developers don't need to ignore
+    # TODOs in their IDE settings.
+    todo: ignore
   exclude:
     - test/data/**