Merge pull request #3136 from stan-dev/chol2inv-simp #4487

Job	Run time
mix/fun tests 1	2h 13m 46s
mix/fun tests 2	1h 33m 29s
prim and rev tests	2h 45m 52s
fwd tests and non-fun mix tests	58m 23s
	7h 31m 30s

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