STL: Replace SFINAE with concepts in C++20-only code when all supported C++20 frontends support concepts

[StephanTLavavej]

See #395. After that simple, mechanical change, we should audit all C++20-only code and replace SFINAE usage with concepts where possible. (#309 is an example where concepts should be used.)

[frederick-vs-ja]

I think more than 90% of C++20+-only codes are done after #4819.

Remaining parts:

• tuple constructors: blocked on LLVM-59827, will be possibly unblocked after requiring Clang 18.
• priority_queue deduction guides: blocked on CWG-2369, which is not yet implemented by MSVC and Clang.
• zoned_time constructors: blocked on LWG-4067 (IMO), the constraints are weird currently.

Anything missing?

[StephanTLavavej]

For priority_queue, I've filed VSO-2452930 "Implement CWG-2369: Ordering between constraints and substitution" for MSVC, and llvm/llvm-project#122423 is in progress for Clang.

llvm/llvm-project#59827 has been merged and we're requiring Clang 19 now - is tuple unblocked?