[C API] Prevent nullptr dereferences in C API for counting attributes.

See https://reviews.llvm.org/D26392

Patch by @maleadt

llvm-svn: 287044
diff --git a/llvm/tools/llvm-c-test/llvm-c-test.h b/llvm/tools/llvm-c-test/llvm-c-test.h
index 0d1ade0..0401f1d 100644
--- a/llvm/tools/llvm-c-test/llvm-c-test.h
+++ b/llvm/tools/llvm-c-test/llvm-c-test.h
@@ -52,6 +52,10 @@
 // diagnostic.c
 int llvm_test_diagnostic_handler(void);
 
+// attributes.c
+int llvm_test_function_attributes();
+int llvm_test_callsite_attributes();
+
 #ifdef __cplusplus
 }
 #endif /* !defined(__cplusplus) */