<flat_set>: Fix some significant bugs caught by latest libcxx tests

[frederick-vs-ja]

Tested with llvm/llvm-project@0b2ab11.

• Fixes narrowing conversion in std::inplace_merge due to assumption that the every wrapped iterator type has the same difference type as the unwrapped version.
• Removes uses of optional-for-sequence-containers cbegin/cend.
• Properly constrains constructors and deduction guides of flat_(multi)set.

Many libcxx tests seem to be buggy - e.g. some are testing uses with underlying containers that lack assign or insert_range, some are test moving comparators, some are testing bogus emplace member functions.

[StephanTLavavej]

Awesome, thank you! 😻