[caffe2] fix build failures in optimized builds under clang
Summary:
There are various possible approaches, but the approach chosen minimizes disruption to source control blame.
Addresses:
```
error: Function _ZN23FunctionalTest_Pad_Test8TestBodyEv is too big to optimize [-Werror,-Wignored-optimization-argument]
```
Test Plan: buck2 build mode/opt caffe2/test/cpp/api:functional
Reviewed By: jamesr66a
Differential Revision: D34027291
fbshipit-source-id: 9dfd771ad56d3d4bc0d41b38b04654c8dae7c006
(cherry picked from commit d43b5a7ed6224192fff8ad1215a572b333ec5f97)
