Fix pretty-printers not working when constexpr variables are optimized away

[k3DW]

Closes #314

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (59e2a56) to head (495f1ab).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #315      +/-   ##
===========================================
- Coverage    98.24%   98.23%   -0.01%     
===========================================
  Files          161      161              
  Lines        22224    22224              
===========================================
- Hits         21833    21832       -1     
- Misses         391      392       +1     

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

... and 1 file with indirect coverage changes

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