[sledge] Set stderr to unbuffered in debug mode
[sledge] Set stderr to unbuffered in debug mode

Summary:
Set stderr to unbuffered when in debug mode, so that debug tracing
output does not get lost in a buffer in case of a crash. This is
slower, so perhaps we will want to add a separate flag to control
buffering.

Reviewed By: bennostein

Differential Revision: D38428834

fbshipit-source-id: db7ddb99f2a67a6c348deda073ea8082035b139a

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