Revert "Add nonnull; use it for atomics"

That's r333325, as well as follow-up "Fix GCC handling of ATOMIC_VAR_INIT"
r333327.

Marshall asked to revert:

Let's have a discussion about how to implement this so that it is more friendly
to people with installed code bases. We've had *extremely* loud responses to
unilaterally adding warnings - especially ones that can't be easily disabled -
to the libc++ code base in the past.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@333351 91177308-0d34-0410-b5e6-96231b3b80d8