MFC r312538:
Addition of clang nullability qualifiers.

For consistency with the qualifiers added in r310977, define a new
qualifier _Null_unspecified which is also defined in clang 3.7+.

Add two new macros:
NULLABILITY_PRAGMA_PUSH
NULLABILITY_PRAGMA_POP

These are for use in headers when we want avoid noisy warnings if
some pointers are left without nullability annotations.

These are added with way ahead of their first use to teach the GCC
ports headers of their existance before their first use.