Fix ForwardDiff derivative of NaNMath.pow

[devmotion]

Fixes #1399.

The PR basically copies JuliaDiff/ForwardDiff.jl#717

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 6.42%. Comparing base (6202fa4) to head (e3bf321).
Report is 52 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1400      +/-   ##
=========================================
+ Coverage    3.95%   6.42%   +2.46%     
=========================================
  Files          51      51              
  Lines        4824    4840      +16     
=========================================
+ Hits          191     311     +120     
+ Misses       4633    4529     -104     

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[ChrisRackauckas]

Looks like there's still an ambiguity?

[devmotion]

That's unrelated AFAICT (and not intended to be fixed by the PR). I opened JuliaSymbolics/SymbolicUtils.jl#690 because I thought it was caused by SymbolicUtils. Maybe something has to be defined in Symbolics instead...

[devmotion]

This PR should be ready for review. The remaining test failures are the same as on the master branch, and seem unrelated.