[mlir][PDLL] Don't use the result of `Constraint::getDefName()` when uniquing

In the case of anonymous defs this may return the name of the base def class,
which can lead to two different defs with the same name (which hits an assert).
This commit adds a new `getUniqueDefName` method that returns a unique name
for the constraint.

Differential Revision: https://reviews.llvm.org/D124074