ada: Accessibility error incorrectly flagged on call within Pre'Class expression

ada: Accessibility error incorrectly flagged on call within Pre'Class expression

The compiler was wrongly reporting an error on a function call within
a Pre'Class expression when a formal of the aspect's subprogram was
passed to an aliased formal. This occurred due to the call appearing
with the return statement of the wrapper function created for the
precondition, but the accessibility error check should only be done
for return statements that appear in the source program.

gcc/ada/

	* sem_ch4.adb (Analyze_Call): Add test of Comes_From_Source on the
	enclosing subprogram's Entity_Id for determining whether to
	perform the compile-time accessibility check on actuals passed to
	aliased formals in a function call occurring within a return
	statement. That test excludes cases where the call occurs within
	the return statement of a Pre'Class wrapper function.