[-Wcompletion-handler] Support checks with builtins

It is very common to check callbacks and completion handlers for null.
This patch supports such checks using built-in functions:
  * __builtin_expect
  * __builtin_expect_with_probablity
  * __builtin_unpredictable

rdar://73455388

Differential Revision: https://reviews.llvm.org/D96268