[hal] Add [[nodiscard]] to HAL_WaitForNotifierAlarm() (#3085)

As this is a C-compatible header, add a WPI_NODISCARD macro for
compiler-specific C-compatible nodiscard attributes.

wpiutil/src/main/native/include/wpi/nodiscard.h

@@ -0,0 +1,21 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#ifndef WPIUTIL_WPI_NODISCARD_H_
+#define WPIUTIL_WPI_NODISCARD_H_
+
+// This should only be used on APIs that can be compiled as C or C++. If the API
+// is C++ only, use [[nodiscard]] instead.
+#ifndef WPI_NODISCARD
+#ifdef __cplusplus
+#define WPI_NODISCARD [[nodiscard]]
+#elif defined(__GNUC__) || defined(__llvm__)
+#define WPI_NODISCARD __attribute__((warn_unused_result))
+#elif _MSC_VER
+#define WPI_NODISCARD _Check_return_
+#endif
+
+#endif
+
+#endif  // WPIUTIL_WPI_NODISCARD_H_