Codechange: Use std::variant to store string parameter data.

This avoids storing two separate values and makes the test for which type is held clearer.

This replaces use of unique_ptr for conditionally storing a string, and is also used in place of StringParameterBackup.

src/strings_type.h

@@ -88,34 +88,6 @@ enum SpecialStrings {
 	SPECSTR_PRESIDENT_NAME     = 0x70E7,
 };
 
-/** Data that is to be stored when backing up StringParameters. */
-struct StringParameterBackup {
-	uint64_t data; ///< The data field; valid *when* string has no value.
-	std::optional<std::string> string; ///< The string value.
-
-	/**
-	 * Assign the numeric data with the given value, while clearing the stored string.
-	 * @param data The new value of the data field.
-	 * @return This object.
-	 */
-	StringParameterBackup &operator=(uint64_t data)
-	{
-		this->string.reset();
-		this->data = data;
-		return *this;
-	}
-
-	/**
-	 * Assign a copy of the given string to the string field, while clearing the data field.
-	 * @param string The new value of the string.
-	 * @return This object.
-	 */
-	StringParameterBackup &operator=(const std::string_view string)
-	{
-		this->data = 0;
-		this->string.emplace(string);
-		return *this;
-	}
-};
+using StringParameterData = std::variant<uint64_t, std::string>;
 
 #endif /* STRINGS_TYPE_H */