[IR] Delete llvm::Constants using the correct type.

In most cases, this doesn't have much impact: the destructors just call
the base class destructor anyway.  A few subclasses of ConstantExpr
actually store non-trivial data, though. Make sure we clean up
appropriately.

This is sort of ugly, but I don't see a good alternative given the
constraints.

Issue found by asan buildbots running the testcase for D80330.

Differential Revision: https://reviews.llvm.org/D82509