cleanup messages allow list

- Pass around container by reference instead of value
  (avoid unneeded deep-copies)
- Use std::unordered_set instead of std::set (no need to be sorted)
- Rename variable "messages_to_compile" to "messages_allowlist"
  to reduce confusion with other message lists in the code
- Remove raw pointer

Bug: 321668269
Test: presubmit
Change-Id: I43eaf5c1db3d15b066a95f34373211aadf6417b4