[pytorch/tensorexpr] Update use of LLJIT::lookup for LLVM 15
`lookup` will return an address, rather than a symbol, in LLVM 15.
For consistency with the old APIs we can just wrap the address in a fake symbol
and return that, since we only care about the address anyways.
Differential Revision: [D36282406](https://our.internmc.facebook.com/intern/diff/D36282406/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77169
Approved by: https://github.com/jfix71, https://github.com/ZolotukhinM
