[JIT][to-backend] Fix compilation unit and name mangling of generated module (#38679)
Summary:
**Summary**
This commit gets rid of the separate compilation unit that is currently
being created for every backend-specific module generated by
`jit::backend::generateToBackendFn` and mangles the name properly to
allow multiple backend-specific modules to coexist in the same
compilation unit.
**Test Plan**
`python test/test_jit.py TestBackends`
**Fixes**
This pull request fixes part of https://github.com/pytorch/pytorch/issues/37841.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38679
Differential Revision: D21744620
Pulled By: SplitInfinity
fbshipit-source-id: ac85b8ce0d179c057991e9299fd53a4e13ba02a9
