Can't find LPM_CONSTANT in Quartus 17.1 Pro. In Quartus 17.1 Standart it's located in Miscellaneous. In Quartus 17.1 Pro there's no such megafunction there or anywhere else.
It may not be available for Quartus Pro edition. Either you can use CONSTANT symbol in *.bdf file or include the *.v or *.vhd files ( from available 'LPM_CONSTANT' in std.) into the Quartus project.