[sledge] Add __llair_assume convenience macro
[sledge] Add __llair_assume convenience macro

Summary:
A trivial implementation in terms of `__llair_unreachable`, but helps
readability nonetheless.

Differential Revision: D39136454

fbshipit-source-id: 001cab10780314b1848a7f1477bc92af3dbcd808

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