AMReX Enum: Underlying Value (#4616)

## Summary

Sometimes one needs the underlying (u)int value of an `enum class`,
e.g., when building bitmasks. This adds a helper function to
`AMREX_ENUM`.

## Additional background

BLAST-ImpactX/impactx#1104

---------

Co-authored-by: Weiqun Zhang <[email protected]>

Author: ax3l

Src/Base/AMReX_Enum.H

@@ -140,6 +140,17 @@ namespace detail
     {
         return std::string(ET::class_name);
     }
+
+    /** Return the underlying (u)int of an enum value
+     *
+     * Useful when building bitmasks.
+     */
+    template <typename T, typename ET = amrex_enum_traits<T>,
+              std::enable_if_t<ET::value,int> = 0>
+    constexpr auto toUnderlying (T v) noexcept
+    {
+        return static_cast<std::underlying_type_t<T>>(v);
+    }
 }
 
 #define AMREX_ENUM(CLASS, ...) \