ada: Fix crash on nested access-to-subprogram types

ada: Fix crash on nested access-to-subprogram types

This patch fixes a crash on some subprograms with anonymous
access-to-subprogram parameters by removing delayed freezing of
subprograms in some cases where it wasn't necessary. The -gnatD output
for itypes is also improved.

gcc/ada/ChangeLog:

	* sem_ch6.adb (Check_Delayed_Subprogram, Possible_Freeze): Restrict
	cases where freezing is delayed.
	* sem_ch6.ads (Check_Delayed_Subprogram): Improve documentation
	comment.
	* sprint.adb (Write_Itype): Improve output.