ada: Allow IN OUT parameters for first parameter of traversal functions

ada: Allow IN OUT parameters for first parameter of traversal functions

In general, functions in SPARK cannot have parameters of mode IN OUT
unless they are annotated with the Side_Effects aspect. Borrowing
traversal functions are special functions which can return a part
of their first parameter as an access-to-variable type. This might not
be allowed in Ada if the parameter is a constant. Allow the first
parameter of borrowing traversal functions to have mode IN OUT.

gcc/ada/ChangeLog:

	* sem_ch6.adb (Analyze_SPARK_Subprogram_Specification):
	Allow the first parameter of functions whose return type is
	an anonymous access-to-variable type to have mode IN OUT.