ada: Improve message for misused implicitly-defined preprocessor symbol.

ada: Improve message for misused implicitly-defined preprocessor symbol.

If the -u option is specified, then otherwise-undefined preprocessor
symbols are implicitly defined to be False. If such an implicitly-defined
symbol is then incorrectly used in a context that requires an integer value,
the resulting error message should not incorrectly state that the symbol is
undefined.

gcc/ada/ChangeLog:

	* prep.adb (Expression): Improve error message text when an
	implicitly-defined Boolean-valued symbol is used in a context that
	requires an integer value.