check functions for AccessSpreader
check functions for AccessSpreader

Summary: Add some functions for looking at the code generated by `AccessSpreader<>::current()` and `AccessSpreader<>::cachedCurrent()`.

Differential Revision: D27747508

fbshipit-source-id: 0bc44938e810ec76ead50b6d4c1ade4aec9f8282

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly