[Ada] Implement new switch -gnatw.g

[Arnaud Charlet <charlet at adacore dot com>]

This patch implements a new switch -gnatw.g which implements the set
of warnings corresponding to the use of the -gnatg switch. This allows
compilations using the standard set of GNAT warnings without setting
the -gnatg switch itself. In the long run, we intend to restrict the
use of -gnatg to compiling the run time. At the same time, we clean up
the implementation of warnings in -gnatg mode, adding a new routine for
that purpose to sem_warn and removing an odd kludge in sem_ch10 which
enabled Implementation_Unit_Warnings in -gnatg mode, and then tested
GNAT_Mode explicitly to disable the warning.

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

2009-07-15  Robert Dewar  <dewar@adacore.com>

	* switch-c.adb, sem_ch10.adb, sem_warn.adb, sem_warn.ads: Implement
	new switch -gnatw.g.
	(Set_GNAT_Mode_Warnings): New procedure.

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