The ASAN report fetching code had two latent bugs:

1) It was forward declaring functions without 'extern "C"'.  That used to work
   but only because of another bug in how we passes symbol only function names to the
   compiler and stopped working recently.
2) These forward declarations were in the body of the User Expression, and they actually
   need to go in the prefix file.

<rdar://problem/24177689>

llvm-svn: 257852