Generalize build_variable for Variable(s)ConstrainedOnCreation

[pulsipher]

Closes #2594.

[codecov]

Codecov Report

Merging #2595 (75c02f0) into master (8062654) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2595      +/-   ##
==========================================
+ Coverage   93.45%   93.47%   +0.01%     
==========================================
  Files          44       44              
  Lines        5533     5547      +14     
==========================================
+ Hits         5171     5185      +14     
  Misses        362      362              

Impacted Files	Coverage Δ
src/macros.jl	92.96% <ø> (ø)
src/sd.jl	92.20% <ø> (ø)
src/sets.jl	90.90% <ø> (+0.43%)	⬆️
src/constraints.jl	95.00% <0.00%> (+0.02%)	⬆️
src/nlp.jl	93.44% <0.00%> (+0.04%)	⬆️
src/JuMP.jl	83.54% <0.00%> (+0.51%)	⬆️

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