Add pg_noinline macro to c.h.
Forcing a function not to be inlined can be useful if it's the
slow-path of a performance critical function, or should be visible in
profiles to allow for proper cost attribution.
Author: Andres Freund
Discussion: https://postgr.es/m/
20170914061207.zxotvyopetm7lrrp@alap3.anarazel.de