add unsigned-integer-overflow suppressions to Hash.h
add unsigned-integer-overflow suppressions to Hash.h

Summary:
I'd like to enable UBSAN's unsigned-integer-overflow check. It's not an undefined behavior, but it is often unintentional.

This adds suppressions to folly/hash where unsigned-integer-overflow is expected due the the nature of operations.

Reviewed By: yfeldblum

Differential Revision: D19217819

fbshipit-source-id: 1a468018823e0ac1d01c3ae889e38e9d42bdd8b7

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