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.

llvm-svn: 199626
diff --git a/clang/test/Sema/nonnull.c b/clang/test/Sema/nonnull.c
index 3c56075..f1ba482 100644
--- a/clang/test/Sema/nonnull.c
+++ b/clang/test/Sema/nonnull.c
@@ -32,4 +32,10 @@
   baz3(0); // no-warning
 }
 
+void test_void_returns_nonnull() __attribute__((returns_nonnull)); // expected-warning {{'returns_nonnull' attribute only applies to return values that are pointers}}
+int test_int_returns_nonnull() __attribute__((returns_nonnull)); // expected-warning {{'returns_nonnull' attribute only applies to return values that are pointers}}
+void *test_ptr_returns_nonnull() __attribute__((returns_nonnull)); // no-warning
+
 int i __attribute__((nonnull)); // expected-warning {{'nonnull' attribute only applies to functions, methods, and parameters}}
+int j __attribute__((returns_nonnull)); // expected-warning {{'returns_nonnull' attribute only applies to functions and methods}}
+