ada: VAST: Check basic tree properties

ada: VAST: Check basic tree properties

Miscellaneous improvements to VAST. Mostly debugging improvements.

Move the call to VAST from Frontend to Gnat1drv, because
there is code AFTER the call to Frontend that notices
certain errors, and disables the back end. We want VAST
to be enabled only when the back end will be called.
This is needed to enable Check_Error_Nodes, among other
things.

gcc/ada/ChangeLog:

	* frontend.adb: Move call to VAST from here...
	* gnat1drv.adb: ...to here.
	* vast.ads (VAST_If_Enabled): Rename main entry point of VAST from
	VAST to VAST_If_Enabled.
	* vast.adb: Miscellaneous improvements. Mostly debugging
	improvements. Also enable Check_Error_Nodes. Also add checks:
	Check_FE_Only, Check_Scope_Present, Check_Scope_Correct.
	* debug.ads: Minor comment tweaks. The comment, "In the checks off
	version of debug, the call to Set_Debug_Flag is always a null
	operation." appears to be false, so is removed.
	* debug.adb: Minor: Remove some code duplication.
	* sinfo-utils.adb (nnd): Add comment warning about C vs. Ada
	confusion.