INOX (which is enabled via -gnatX) supports composite case-statement selectors.
As a temporary measure, simplify the coverage-related compile-time checks
for such case statements via two changes: an others choice is always
required for such a case statement, and no legality checks relating to
overlapping of case choices are performed.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_case.adb: Define a new Boolean constant,
	Simplified_Composite_Coverage_Rules, initialized to True. Setting
	this constant to True has two effects: 1- Representative value
	sets are not fully initialized - this is done to avoid capacity
	problems, as well as for performance. 2- In
	Check_Case_Pattern_Choices, the only legality check performed is a
	check that a "when others =>" choice is present.