[libc++][flat_set] LWG3751, LWG3774 (#153934)
- LWG3751: Missing feature macro for `flat_set`
Implemented in LLVM21:
https://github.com/llvm/llvm-project/commit/7013b51548c0bd2c7e5564735c44506909a2f8dc
Closes #105021
- LWG3774: `<flat_set>` should include `<compare>`
Implemented in LLVM21:
https://github.com/llvm/llvm-project/commit/2f1416bbcde898d65e9762e6ce498bb0121e4610
https://github.com/llvm/llvm-project/blob/684797b6446073b5afd23022449aeecef84c404c/libcxx/include/flat_set#L77
Closes #105036
