Adopt `TrailingObjects` convenience API that was added in
https://github.com/llvm/llvm-project/pull/138970 in LLVM and MLIR code.

NumBits should be less than 20 so using an unsigned instead of size_t should be OK

Remove unused variable to fix build failures from bot.

Implement TrieRawHashMap can be used to store object with its associated
hash. User needs to supply a strong hashing function to guarantee the
uniqueness of the hash of the objects to be inserted. A hash collision
is not supported and will lead to error or failed to insert.

TrieRawHashMap is thread-safe and lock-free and can be used as
foundation data structure to implement a content addressible storage.
TrieRawHashMap owns the data stored in it and is designed to be:
* Fast to lookup.
* Fast to "insert" if the data has already been inserted.
* Can be used without lock and doesn't require any knowledge of the
participating threads or extra coordination between threads.

It is not currently designed to be used to insert unique new data with
high contention, due to the limitation on the memory allocator.