Refactor: Move flag descriptions to inline constexpr in header

This change refactors how feature flag names and descriptions are defined. Instead of using `extern const char` declarations in `ios_chrome_flag_descriptions.h` and definitions in `ios_chrome_flag_descriptions.cc`, the strings are now defined directly in the header using `inline constexpr char`.

This eliminates the need for the separate `ios_chrome_flag_descriptions.cc` file, simplifying the build and reducing potential for ODR violations. The `create_feature_flag.py` script has been updated to reflect this change.

Change-Id: I00222c1b74c2c279ce189746d7bb66b832708529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7816360
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1626121}