Wire up basic parser/sema support for attribute 'returns_nonnull'.

Wire up basic parser/sema support for attribute 'returns_nonnull'.

This attribute is supported by GCC.  More generally it should
probably be a type attribute, but this behavior matches 'nonnull'.

This patch does not include warning logic for checking if a null
value is returned from a function annotated with this attribute.
That will come in subsequent patches.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199626 91177308-0d34-0410-b5e6-96231b3b80d8