[DFSan] New custom wrappers

Added the following custom wrappers:
  strstr
  strrchr
  memchr
  nanosleep
  socketpair

Tweaked a couple of existing wrappers: if a wrapper returns a pointers derived
from an input pointer, then return the label of the input pointer

Sorted tests invocation

Differential Revision: D2354

llvm-svn: 197601