Only generate above-Uni warning for \p{}, \P{}

Only generate above-Uni warning for \p{}, \P{}

This warning was being generated inappropriately during some internal
operations, such as parsing a program; spotted by Tom Christiansen.

The solution is to move the check for this situation out of the common
code, and into the code where just \p{} and \P{} are handled.

As mentioned in the commit's perldelta, there remains a bug
[perl #114148], where no warning gets generated when it should