Disentangle the LUD structs from the regular GWP-ASan structs - CL 3/3

The change will allow Lightweight UAF Detector to run independently of
the main GWP-ASan mode without having to make the validation function
logic more complex.

This CL introduces the new detector-specific feature flag.

Bug: 1418006
Change-Id: Ib39a20c22ea6105f1bd9a5138e109447a9bfc88c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4801534
Commit-Queue: Sergei Glazunov <glazunov@google.com>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1192930}