[Ada] Make -gnatU and -gnatw.d the default

[Pierre-Marie de Rodat]

-gnatU prepends `error:` to error messages, enabling it by default makes
error messages more consistent with warnings.  -gnatw.d tags messages
with the flag that caused them.

As users might need to switch back to the previous behavior, the
`-gnatd_U` flag is introduced to do exactly that.

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

gcc/ada/

	* debug.adb: Document -gnatd_U as taken.
	* err_vars.ads (Warning_Doc_Switch): Set to True.
	* errout.ads (Errout): Update documentation.
	* gnat1drv.adb (Adjust_Global_Switches): React to -gnatd_U.
	* hostparm.ads (Tag_Errors): Set to True.
	* opt.ads (Unique_Error_Tag): Document -gnatd_U.
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