ipc_helpers: Add PushEnum() member function to ResponseBuilder

Allows pushing strongly-typed enum members without the need to always
cast them at the call sites.

Note that we *only* allow strongly-typed enums in this case. The reason
for this is that strongly typed enums have a guaranteed defined size, so
the size of the data being pushed is always deterministic. With regular
enums this can be a little more error-prone, so we disallow them.

This function simply uses the underlying type of the enum to determine
the size of the data. For example, if an enum is defined as:

enum class SomeEnum : u16 {
  SomeEntry
};

if PushEnum(SomeEnum::SomeEntry); is called, then it will push a
u16-size amount of data.

src/core/hle/ipc_helpers.h

@@ -174,6 +174,25 @@ public:
     template <typename First, typename... Other>
     void Push(const First& first_value, const Other&... other_values);
 
+    /**
+     * Helper function for pushing strongly-typed enumeration values.
+     *
+     * @tparam Enum The enumeration type to be pushed
+     *
+     * @param value The value to push.
+     *
+     * @note The underlying size of the enumeration type is the size of the
+     *       data that gets pushed. e.g. "enum class SomeEnum : u16" will
+     *       push a u16-sized amount of data.
+     */
+    template <typename Enum>
+    void PushEnum(Enum value) {
+        static_assert(std::is_enum_v<Enum>, "T must be an enum type within a PushEnum call.");
+        static_assert(!std::is_convertible_v<Enum, int>,
+                      "enum type in PushEnum must be a strongly typed enum.");
+        Push(static_cast<std::underlying_type_t<Enum>>(value));
+    }
+
     /**
      * @brief Copies the content of the given trivially copyable class to the buffer as a normal
      * param