[asan] Replace vfork with fork.

Summary:
vfork is not ASan-friendly because it modifies stack shadow in the
parent process address space. While it is possible to compensate for that with, for example,
__asan_handle_no_return before each call to _exit or execve and friends, simply replacing
vfork with fork looks like by far the easiest solution.

Posix compliant programs can not detect the difference between vfork and fork.

Fixes https://github.com/google/sanitizers/issues/925

Reviewers: kcc, vitalybuka

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D44587

llvm-svn: 327752