[Ada] Further work on pre/post aspects

[Arnaud Charlet <charlet at adacore dot com>]

This patch implements a new warning switch -gnatw.l that causes
listing of inherited Pre/Post aspects, this is on by default.
It also does a general cleanup of error message handling for
Pre/Post attributes, and there is a complete implementation of
inherited Pre'Class attributes.

Still to be done in this AI:

Handle visibility right, the GNAT rules for pragmas do not
quite match the RM rules for Pre/Post aspects.

Deal with entries (not clear yet what is involved)

Fix error in handling inherited preconditions, these still
don't work, but it is helpful to checkin the current sources
to further investigate the problem.

Resolve the issue of 'Old restrictions

Tested on x86_64-pc-linux-gnu, committed on trunk

2010-10-12  Robert Dewar  <dewar@adacore.com>

	* errout.ads, erroutc.adb: The # insertion now handles from in place of
	at.
	* exp_prag.adb (Expand_Pragma_Check): Suppress generated default
	message if new switch Exception_Locations_Suppressed is set.
	(Expand_Pragma_Check): Revised wording for default message for case
	of precondition or postcondition.
	* namet.ads, namet.adb (Build_Location_String): New procedure.
	* opt.ads (List_Inherited_Pre_Post): New flag.
	* sem_ch12.adb (Analyze_Generic_Subprogram_Declaration): Add call to
	list inherited pre/post aspects.
	* sem_ch13.adb (Analyze_Aspect_Specification): Improve generation of
	messages for precondition/postcondition cases.
	* sem_ch6.adb (Process_PPCs): General cleanup, and list inherited PPC's
	if flag List_Inherited_Pre_Post is set True.
	(Process_PPCs): Add initial handling for inherited preconditions
	(List_Inherited_Pre_Post_Aspects): New procedure
	* sem_ch6.ads (List_Inherited_Pre_Post_Aspects): New procedure
	* sem_disp.adb (Inherited_Subprograms): New function
	* sem_disp.ads (Inherited_Subprograms): New function
	* sem_prag.adb (Check_Duplicate_Pragma): Clean up handling of
	pre/postcondition.
	(Check_Precondition_Postcondition): Check for inherited aspects
	* sem_warn.adb: Process -gnatw.l/w.L setting List_Inherited_Pre_Post
	* sinfo.ads, sinfo.adb (Split_PPC): New flag.
	* sinput.ads, sinput.adb (Build_Location_String): New function.
	* usage.adb: Add line for -gnatw.l/-gnatw.L

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