Fix DifferentiateVarDecl for constructors in reverse mode

[vaithak]

This fixes the error in #627 related to invalid operands in a binary expression.

A very minimal example that was failing earlier:

class Experiment {
   
public:
    Experiment(int n=0){}
};

double f(double x, int n) {
  Experiment E(n);
  return 0.0; 
}

int main() {
  auto f_dx = clad::gradient(f, "x");
}

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[codecov]

Codecov Report

Merging #630 (3acc860) into master (71dd694) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 88.88%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #630   +/-   ##
=======================================
  Coverage   94.04%   94.05%           
=======================================
  Files          43       43           
  Lines        6230     6236    +6     
=======================================
+ Hits         5859     5865    +6     
  Misses        371      371           

Files Changed	Coverage Δ
lib/Differentiator/StmtClone.cpp	51.35% <0.00%> (ø)
tools/ClangBackendPlugin.cpp	66.66% <ø> (ø)
include/clad/Differentiator/Compatibility.h	91.04% <100.00%> (+0.27%)	⬆️
include/clad/Differentiator/VisitorBase.h	100.00% <100.00%> (ø)
lib/Differentiator/ReverseModeVisitor.cpp	95.77% <100.00%> (+<0.01%)	⬆️

Files Changed	Coverage Δ
lib/Differentiator/StmtClone.cpp	51.35% <0.00%> (ø)
tools/ClangBackendPlugin.cpp	66.66% <ø> (ø)
include/clad/Differentiator/Compatibility.h	91.04% <100.00%> (+0.27%)	⬆️
include/clad/Differentiator/VisitorBase.h	100.00% <100.00%> (ø)
lib/Differentiator/ReverseModeVisitor.cpp	95.77% <100.00%> (+<0.01%)	⬆️

[vgvassilev]

LGTM!

[vgvassilev]

The codecov report seems invalid. Let’s merge it.