cmTarget: Deprecate the LOCATION target property with a policy.
The final location and name of a build-target is not determined until generate-time. However, reading the LOCATION property from a target is currently allowed at configure time. Apart from creating possibly-erroneous results, this has an impact on the implementation of cmake itself, and prevents some major cleanups from being made. Disallow reading LOCATION from build-targets with a policy. Port some existing uses of it in CMake itself to use the TARGET_FILE generator expression.