[pulse] avoid false positives involving `folly::fbstring`
[pulse] avoid false positives involving `folly::fbstring`

Summary:
That class does some complicated accounting of memory that depends on
whether the string is "small", "medium" or "large". In the latter case
it does its own ref-counting and copy-on-write to save memory, and that
trips up pulse. Pretending all strings are small avoids that issue.

Reviewed By: skcho

Differential Revision: D18909030

fbshipit-source-id: 1c14d909b

jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer