ada: VAST: create treewalker

ada: VAST: create treewalker

Walks all trees (not just the main unit), deals with switches and
flags. Doesn't check much of anything yet (asserts that "unused" nodes
are not present).

Move decisions (what tree(s) to check, what switches enable checking)
from the caller to the body of VAST.

gcc/ada/ChangeLog:

	* vast.adb: Initial implementation.
	* vast.ads: Rename procedure. Remove parameter; body should decide
	what to do.
	* lib.ads (ipu): Minor: Rewrite comment for brevity, and because
	of an inconvenient misspelling.
	(Num_Units): Not used; remove.
	(Remove_Unit): Minor: Remove "Currently" (which was current a decade
	ago from) comment.
	* lib.adb (Num_Units): Not used; remove.
	* debug_a.adb (Debug_A_Entry): Fix bug: Use Write_Name_For_Debug,
	so this won't crash on the Error node.
	* debug.adb: Document -gnatd_V and -gnatd_W compiler switches.
	* exp_ch6.adb (Validate_Subprogram_Calls): Remove redundant check for
	Serious_Errors_Detected. (We turn off code gen when errors are
	detected.)
	* frontend.adb: Move decisions into VAST body.
	* namet.ads (Present): Remove unnecessary overriding; these are
	inherited by the derived types.
	* namet.adb (Present): Likewise.