[IR] Added operator delete to subclasses of User to avoid UB

Several subclasses of User override operator new without also overriding
operator delete. This means that delete expressions fall back to using
operator delete of the base class, which would be User. However, this is
only allowed if the base class has a virtual destructor which is not the
case for User, so this is UB.

See also [expr.delete] (3) for the exact wording.

This is actually detected in some cases by GCC 11's
-Wmismatched-new-delete now which is how I found this error.

Differential Revision: https://reviews.llvm.org/D103143