ada: Fix assertion failure on very peculiar enumeration type

ada: Fix assertion failure on very peculiar enumeration type

The compiler currently does not support the combination of a representation
clause on an enumeration type with a size clause whose value is greater than
the size of the largest machine scalar supported by the target.

Given that such a type would have little practical value, this change causes
the compiler to give a proper error message instead of aborting.

gcc/ada/

	* freeze.adb (Freeze_Enumeration_Type): Give an error on a type with
	both representation clause and too large size.