iterator-checker: Update annotation system

This new annotation system is based on the fact that both functions and types can be annotated, rather than just functions.
This gives us more flexibility and makes annotations easier to write and understand.

The atomic annotation is `Annotation` while their aggregation is a vector of them (aliased as `Annotations`).
Given a function (both member and non-member), in order to perform our analysis, it is required to have annotations about its declaration, its return type and its arguments types. All of these are grouped under `GroupedFunctionAnnotation`.

Change-Id: I6cae65b8843b48dd5181dcae351ff851fccb18e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5607767
Reviewed-by: Paul Semel <paulsemel@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1319654}