Logically revert commit 390a7d86 (cmTargetPropertyComputer: Implement
GetProperty without cmMakefile, 2016-10-13, v3.8.0-rc1~445^2~9).
It relied on using `cmListFileBacktrace` to get a scope in which to
look up policies.

This does remove a backtrace from `LOCATION` property errors at generate
time, but the backtrace we reported before was incorrect.  It pointed at
the addition of a target, not to the reference to the property.