Add `-Wsign-compare` to list of clang flags
It caused a number of internal only compilation failures, for example
see:
https://github.com/pytorch/pytorch/pull/74425#issuecomment-1075476438
and https://github.com/pytorch/pytorch/pull/74542#issuecomment-1083518880
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75085
Approved by: https://github.com/ngimel, https://github.com/albanD
