[Ada] improve handling of warnings/errors

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

The form of the Warnings pragma that suppresses specific messages
does not work for warnings coming from gigi/backend. The reason is
that Errout.Finalize was calling Validate_Specific_Warnings before
the back end was called. The symptom was that the warning was
properly suppressed, but a spurious warning that the pragma had
not suppressed any warnings was generated.

The fix embodied in this patch is to introduce a parameter Last_Call
for Finalize that is False for all calls except the last, and True
for the last call. Validate_Specific_Warnings is then called only
for the last call.

gnat.dg/warn2.adb should compile quietly with no warnings.

2007-08-14  Robert Dewar  <dewar@adacore.com>

	* comperr.adb: Fix problem with suppressing warning messages from gigi

	* erroutc.ads, erroutc.adb, errout.ads, 
	errout.adb (Write_Eol): Remove trailing spaces before writing the line
	(Write_Eol_Keep_Blanks): New procedure to write a line, including
	possible trailing spaces.
	(Output_Source_Line): Call Write_Eol_Keep_Blanks to output a source line
	Fix problem with suppressing warning messages from back end
	Improve handling of deleted warnings

	* gnat1drv.adb: 
	Fix problem with suppressing warning messages from back end
	Handle setting of Static_Dispatch_Tables flag.

	* prepcomp.adb: 
	Fix problem with suppressing warning messages from back end

	* exp_intr.adb: Improve handling of deleted warnings

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