…ly (#161072)

It seems that not all files in the Flutter repository are compatible
with clangd. This change improves the clangd invocation in CI to choose
files that are known to work with clangd.

Reason for the change:

In #161012, I'm trying to update
GN to the latest version. However, this change made the order of files
in the compile_commands.json. This caused [the clangd check to
fail](https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20clangd/10395/overview)
because `engine/src/flutter/tools/clangd_check/bin/main.dart` currently
chooses the first file in the compile_commands.json, and the newly
chosen file
(`engine/src/flutter/third_party/skia/modules/skparagraph/src/Decorations.cpp`)
is not compatible with clangd. To fix this, I'm making the test choose
the file that is known to work with clangd.
(`gen/flutter/assets/_fl__fl_assets_fixtures.cc`)


## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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Co-authored-by: Jonah Williams <[email protected]>

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