Look for the file named unisim_VCOMP.vhd. It has all the component declarations of the Xilinx primitives.
For ISE, this is located at
[install dir]\[version]\ISE_DS\ISE\vhdl\src\unisims
For Vivado,
[install dir]\[version]\data\vhdl\src\unisims
Note, for Vivado, if you have VHDL that has primitives from older devices, then use unisim_retarget_VCOMP.vhd, which retargets old primitives to 7 Series and Ultrascale equivalents. However, there are some cases when retargeting certain primitives is not possible.
If your code is for 7 Series and Ultrascale families, I wouldn't worry about the "retarget" file.