Adds @enum.unique decorator support

[sobolevn]

Right now it uses ast to tell which values are non unique.
It does not support function that return Literal values, Final constants, etc.
But, it works!

After #11247 is merged, I am going to add the support for Literal and Final values.

Refs #11248

Closes #10858

[sobolevn]

Rebased!

cc @JukkaL