Use LLVMSymbolizer directly for unwind inside fbcode (#108800)
Using LLVMSymbolizer directly avoids having to call fork which has caused timeouts in some circumstances.
Differential Revision: [D49070589](https://our.internmc.facebook.com/intern/diff/D49070589/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108800
Approved by: https://github.com/aaronenyeshi
