[libc++][doc] Marks LWG3257 as complete (#77237)

The macros were already updated
- __cpp_lib_string_view in 466df171
- __cpp_lib_array_constexpr in 77b9abfc

Based on the dates of the commit and that
 P0858 "Constexpr iterator requirements"
was completed in LLVM 12, set this issue as completed in the same
version.

Completes
- LWG3257 Missing feature testing macro update from P0858