More robust mangling (#31978)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31978
Currently we keep a `mangleIndex_` that's intenral to compilation unit and
just increment the index when we found the original name is mangled, this doesn't
guarantee the new name is not defined.
This PR fixes the problem by querying whether the new name is defined or not.
fixes: https://github.com/pytorch/pytorch/issues/31268
Test Plan:
fixes the issue
Imported from OSS
Differential Revision: D19350535
fbshipit-source-id: fe3262b2838d4208ab72e2cd4a5970b3a792ae86
