Fix constexpr of gregorian::date::date(special_values) to improve perf

[p12tic]

Fixes #121

Turns out even when all functions without the library are prefixed with constexpr, GCC up to at least v10 does not consider this constructor as constexpr at least when generating code.

BOOST_CXX14_CONSTEXPR explicit date(special_values sv):
      date_time::date<date, gregorian_calendar, date_duration>(date_rep_type::from_special(sv))
    {
      if (sv == min_date_time)
      {
        *this = date(1400, 1, 1);
      }
      if (sv == max_date_time)
      {
        *this = date(9999, 12, 31);
      }
    }

The culprit is the assignment to *this. Refactoring the code to initialize the instance just once improves code generation significantly. Note that clang since at least v11 does not exhibit this problem.

[codecov[bot]]

Codecov Report

Merging #214 (a8478d4) into develop (f972dc9) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #214   +/-   ##
========================================
  Coverage    91.74%   91.75%           
========================================
  Files           77       77           
  Lines         4823     4827    +4     
========================================
+ Hits          4425     4429    +4     
  Misses         398      398           

Impacted Files	Coverage Δ
include/boost/date_time/gregorian/greg_date.hpp	100.00% <100.00%> (ø)
...boost/date_time/posix_time/posix_time_duration.hpp	100.00% <0.00%> (ø)

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[JeffGarland]

Looks good - thanks!