Merge pull request #3110 from stan-dev/fix/pow-overload-resolution #4391

Job	Run time
fwd tests and non-fun mix tests	1h 3m 4s
mix/fun tests 1	2h 13m 1s
mix/fun tests 2	1h 35m 50s
prim and rev tests	2h 53m 56s
	7h 45m 51s

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