Bugfix/issue 1476 lkj corr cholesky fvar

[rtrangucci]

Summary:

Allows for fvar instantiation of lkj_corr_cholesky.

Intended Effect:

Admits higher-order autodiff of lkj_corr_cholesky density

How to Verify:

./runTests.py test/unit/math/mix/mat/prob/lkj_corr_test.cpp

Side Effects:

None.

Documentation:

None.

Reviewer Suggestions:

Anyone

[stan-buildbot]

Build Log
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Starting build job Math Pull Request - Doc - Doxygen.
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