update value_of_rec to return a reference for varmat types

[SteveBronder]

Summary

Just a small bugfix for var matrix types to return back a reference when calling value_of_rec() on them. Fixes the error upstream in stanc3 stan-dev/stanc3#955

Tests

No new tests

Side Effects

value_of_rec(var_value<Eigen::MatrixXd> x) now returns a reference to the value instead of a copy

Release notes

Checklist

• Math issue #(issue number)

• Copyright holder: Steve Bronder

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.57	3.49	1.02	2.15% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.98	-1.85% slower
eight_schools/eight_schools.stan	0.08	0.09	0.91	-10.29% slower
gp_regr/gp_regr.stan	0.14	0.14	1.02	2.23% faster
irt_2pl/irt_2pl.stan	5.7	5.72	1.0	-0.42% slower
performance.compilation	93.51	90.84	1.03	2.85% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.13	8.12	1.0	0.19% faster
pkpd/one_comp_mm_elim_abs.stan	31.46	32.34	0.97	-2.78% slower
sir/sir.stan	121.18	121.26	1.0	-0.07% slower
gp_regr/gen_gp_data.stan	0.04	0.03	1.04	3.64% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.18	3.02	1.06	5.22% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.43	0.38	1.12	10.93% faster
arK/arK.stan	2.05	2.06	0.99	-0.51% slower
arma/arma.stan	0.23	0.24	0.97	-3.6% slower
garch/garch.stan	0.57	0.59	0.98	-2.22% slower
Mean result: 1.00577733649

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Jenkins Console Log
Blue Ocean
Commit hash: 620158b

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Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix