P0879R0 constexpr For Swapping Functions

[StephanTLavavej]

P0879R0 constexpr For Swapping Functions

LWG-3256 Feature testing macro for constexpr algorithms

This may need #156 is_constant_evaluated().

Feature-test macro as of WG21-N4842 if both #6 WG21-P0202 and this feature are implemented:
#define __cpp_lib_constexpr_algorithms 201806L

[BillyONeal]

I'm starting this and hope to get the <xutility> family constexprized shortly.

[BillyONeal]

p0879r0, those in bold have libc++ tests already present we can use:

• swap_ranges
• iter_swap
• revese
• rotate
• partition
• sort
• partial_sort
• partial_sort_copy
• nth_element
• push_heap
• pop_heap
• make_heap
• sort_heap
• is_heap
• next_permutation
• prev_permutation
• swap (but libc++ doesn't have tests for the constexpr iterators)