[Ada] Revamp analysis of conditional expressions

[Ada] Revamp analysis of conditional expressions

The current implementation is skewed toward the first dependent
expression and does not look into the interpretations of the others if
the first one is not overloaded, which can create spurious ambiguities.
And more precise error messages are now given if the types of the
dependent expressions are not compatible.

gcc/ada/

	* sem_ch4.adb (Analyze_Case_Expression): Rewrite.
	(Analyze_If_Expression): Likewise.
	(Possible_Type_For_Conditional_Expression): New function.
	* sem_type.adb (Specific_Type): Retur the base type in more
	cases.