reformulated SFINAE clauses checking key_type completeness

[joaquintides]

emplace(K&& k, V&& v) overloads were SFINAE-protected for key_type move constructibility, additionally supplemented with a check for key_type completeness to make incomplete_tests pass. SFINAEing on type completeness for an incomplete type T and completing T later on is IFNDR, and GCC 16 detects this and emits a sfinae-incomplete warning.

Solved by transforming key_type move constructibility check into a dependent construct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (7a59597) to head (2da4abc).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #351   +/-   ##
========================================
  Coverage    98.23%   98.23%           
========================================
  Files          161      161           
  Lines        22247    22247           
========================================
  Hits         21855    21855           
  Misses         392      392           

Files with missing lines	Coverage Δ
...de/boost/unordered/detail/foa/concurrent_table.hpp	99.47% <ø> (ø)
include/boost/unordered/detail/foa/core.hpp	99.54% <ø> (ø)
include/boost/unordered/detail/foa/table.hpp	100.00% <ø> (ø)
include/boost/unordered/detail/implementation.hpp	99.23% <ø> (ø)

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