[Ada] Ignore switches for controlling frontend warnings in GNATprove mode

[Pierre-Marie de Rodat]

In the special mode for GNATprove, ignore switches controlling frontend
warnings, like already done for the control of style checks warnings.
Also remove special handling of warning mode in Errout to make up for
the previous division of control between -gnatw (GNAT) and --warnings
(GNATprove).

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

gcc/ada/

	* errout.adb (Record_Compilation_Errors): Remove global
	variable.
	(Compilation_Errors): Simplify.
	(Initialize): Inline Reset_Warnings.
	(Reset_Warnings): Remove.
	* errout.ads (Reset_Warnings): Remove.
	(Compilation_Errors): Update comment.
	* gnat1drv.adb (Adjust_Global_Switches): Ignore all frontend
	warnings in GNATprove mode, except regarding elaboration and
	suspicious contracts.
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