[stable] [analyzer] Don't put folders in the options map if they use the same file as parent

This loop currently adds every folder recursively to the options map, even if it's the same as the parent. This results in a huge number of entries in the map for large workspaces, and that then `AnalysisOptionsMap operator []` enumerates each of those entries calling `folder.contains(file.path)` which turns out to be rather slow (it calls into `Context._pathDirection`) - see https://github.com/dart-lang/sdk/issues/62456#issuecomment-3806088436

With this change, we only ever record the file if it's not the same as the parent, so in a workspace with 5 analysis_options files and 10,000 folders, we now have 5 entries instead of 10,000.

See https://github.com/dart-lang/sdk/issues/62456

Why cherry-pick? This is a large performance improvement for anyone with a sizeable project, or with many directories (related or unrelated, like `node_modules/` directories) in their project.

Risk: low - the fix has been live in main for a while, and users confirm that in the beta channel, they see the improvement.

Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/476122
Change-Id: If9573a0525f95a23c3c78f84c0ddbc821297b1f0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/481920
Reviewed-by: Leaf Petersen <leafp@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>