Ignore TODOs in analysis_server_plugin.

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.

(See also https://dart-review.googlesource.com/c/sdk/+/295662, which
made the corresponding change to the `analysis_server` and `analyzer`
packages, https://dart-review.googlesource.com/c/sdk/+/325121, which
made the corresponding change to the `linter` package, and
https://dart-review.googlesource.com/c/sdk/+/358980, which made the
corresponding change to the `analyzer_plugin` package).

Change-Id: I916c8c7f435d06c1b0bd871ef3829d58c9242218
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410060
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>