Fix logic added by commit 359c500a24 (cmTarget: Raise error if imported
target location is not set, 2020-08-08, v3.19.0-rc1~273^2) to exclude
INTERFACE libraries from the policy. They have no location.
Fixes: #21470
Previously we would synthesize <TARGET_NAME>-NOTFOUND as the location. This
would then end up on the link line and cause build failures.
Policy CMP0110 is added to control this behaviour.
Fixes#19080, #19943.