[caffe2] don't use __FUNCSIG__ when building for Windows with clang (#70561)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70561
When building with strict(er) compiler warnings on Windows, clang complains that `__FUNCSIC__` is a proprietary language extension. When using clang, it seems we can use `__PRETTY_FUNCTION__` instead, like we do on other platforms. This is also in line with the logic on L100:127.
Test Plan: CI
Reviewed By: kalman5
Differential Revision: D33386400
fbshipit-source-id: d45afa92448042ddcd1f68adc7a9ef4643276b31
