[sledge] Add: __llair_choice intrinsic

Summary: To use for nondeterministic choice in tests.

Reviewed By: ngorogiannis

Differential Revision: D23459509

fbshipit-source-id: 318930d02

sledge/model/llair_intrinsics.h

@@ -16,6 +16,8 @@ __attribute__((noreturn)) void __llair_throw(void* thrown_exception);
 /* This models allocation that cannot fail. */
 void* __llair_alloc(unsigned size);
 
+int __llair_choice();
+
 #ifdef __cplusplus
 }
 #endif