tsan: remove real func check from interceptors

If the real function is not intercepted,
we are going to crash one way or another.
The question is just in the failure mode:
error message vs NULL deref. But the message
costs us a check in every interceptor and
they are not observed to be failing in real life
for a long time, also other sanitizers don't
have this check as well (also crash on
NULL deref if that happens).
Remove the check from non-debug mode.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D112540