[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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@197601 91177308-0d34-0410-b5e6-96231b3b80d8