MFC r310977:
Addition of clang nullability qualifiers.

Add two new qualifiers for use by the static checkers:

_Nonnull
The _Nonnull nullability qualifier indicates that null is not a meaningful
value for a value of the _Nonnull pointer type.

_Nullable
The _Nullable nullability qualifier indicates that a value of the
_Nullable pointer type can be null.

These were introduced in Clang 3.7. For more information, see:
http://clang.llvm.org/docs/AttributeReference.html#nonnull

We add these now without using them so that the GCC ports have time to
pick up the header change.

Hinted by: Android Bionic libc
Also seen in: Apple's Libc-1158.20.4