Use unordered_flat_set to speed up pathological compile time

[msooseth]

Description

This speeds up Object::Structure Object::summarizeStructure() const in pathological cases such as #16745 by quite a bit: 39s down to 28.3s to run.

Also removing unused function, topLevelSubObjectNames()

The only tricky part is that unordered_flat_set potentially invalidates iterators when insert/emplace-ing. This is not the case with std::set.

Performance before these changes on #16745 :

        Command being timed: "./solc/solc --bin --experimental --via-ssa-cfg --optimize-yul source.sol"
        User time (seconds): 39.38

Performance after these changes:

        Command being timed: "./solc/solc --bin --experimental --via-ssa-cfg --optimize-yul source.sol"
        User time (seconds): 28.28

Note that I can't just use util::unordered_flat_set<std::string>, the hash function really makes a difference, since the strings are long. I am also constructing the string first and then checking, this way we don't generate them twice.

Note that I can't use std::string const because then the std::move is not effective. This way it's faster (by about 1.5s).

Checklist

• I have read the contributing guidelines and the review checklist
• I have personally reviewed, understood, and tested every change in this PR

AI Disclosure

• No AI tools were used
• I asked an AI to review