add const qualifier to function arguments in LUPSolve() #188

Job	Run time
Build and test g++-9 -	8m 9s
Build and test g++-10 -	8m 20s
Build and test clang++-10 -	6m 57s
Build and test clang++-11 -	7m 15s
Build and test g++-9 - -DPBRT_DBG_LOGGING=True	7m 7s
Build and test g++-10 - -DPBRT_DBG_LOGGING=True	8m 11s
Build and test clang++-10 - -DPBRT_DBG_LOGGING=True	5m 56s
Build and test clang++-11 - -DPBRT_DBG_LOGGING=True	5m 56s
Build and test g++-9 - -DPBRT_FLOAT_AS_DOUBLE=True	9m 9s
Build and test g++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	7m 19s
Build and test clang++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	7m 38s
Build and test clang++-11 - -DPBRT_FLOAT_AS_DOUBLE=True	6m 31s
	1h 28m 28s

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