[Ada] Max_Entry_Queue_Length aspect for protected entries

[Ada] Max_Entry_Queue_Length aspect for protected entries

Allow values of negative one to be accepted as a valid parameter as a
special case.

2019-08-21  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Modify
	documentation to reflect expected behavior.
	* gnat_rm.texi: Regenerate.
	* sem_prag.adb (Analyze_Pragma): Modify handling of pragma
	Max_Entry_Queue_Length to not reject integer values of negative
	one.
	* sem_util.adb (Get_Max_Queue_Length): Add processing for values
	of negative one to fit within the current scheme.

From-SVN: r274774