ada: Constant_Indexing used when context requires a variable

ada: Constant_Indexing used when context requires a variable

In the case of a call with a formal parameter of mode other than "IN"
where the corresponding actual parameter is a generalized indexing
and the indexable container has both Constant_Indexing and Variable_Indexing
aspects specified, the generalized indexing must be interpreted as a
variable indexing, not as a constant indexing. In some cases involving a
call to a prefixed view of a subprogram, this was not handled correctly.
This error results in spurious compile-time error messages saying that
the actual parameter in the call "must be a variable".

gcc/ada/

	* sem_ch4.adb (Constant_Indexing_OK): As a temporary stopgap,
	return False in the case of an unanalyzed prefixed-view call.