_Convert_size() should be conditionally noexcept

[StephanTLavavej]

_Convert_size() is an internal helper function whose sole purpose in life is to throw length_error when a size_t would be truncated to an allocator's narrower size_type (which is unusual, but there's nothing stopping an allocator from having a 32-bit size_type on a 64-bit system).

During earlier maintenance (before the GitHub era), the primary template was incorrectly marked as being unconditionally noexcept. This fixes that bug (which no users noticed), and replaces the explicit specialization for size_t with if constexpr logic.

[StephanTLavavej]

I'm mirroring this to an MSVC-internal PR. Please notify me if any further changes are pushed.