[Ada] Pragma Warning_As_Error works for style warnings

[Ada] Pragma Warning_As_Error works for style warnings

Pragma Warning_As_Error now works for style warnings (messages that
start with "(style)", enabled by -gnaty) the same way it works for
regular warnings enabled by -gnatw.

The following test should fail to build with style checks:

gnat.adc:

pragma Warning_As_Error ("two spaces required");

style.adb:

procedure Style is
   X : Integer;
begin
   null;
   --Hello
end;

gnatmake -q -f -g style.adb -gnaty

should get:

style.adb:2:04: warning: variable "X" is never read and never assigned
style.adb:5:06: error: (style) two spaces required [warning-as-error]
style.adb:6:01: (style) "end Style" required
gnatmake: "style.adb" compilation error

and no executable should be created.

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* errout.adb (Error_Msg_Internal): Set Warn_Err in case of
	Is_Style_Msg.
	* erroutc.adb (Output_Msg_Text): Do Warnings_Treated_As_Errors
	processing and [warning-as-error] modification for style
	messages.  Clean up code, simplify, remove unnecessary block
	statement, add renaming of table entry.
	* erroutc.ads (Warning_Treated_As_Error): Fix comment: no such
	thing as Set_Warning_As_Error.
	* opt.ads: Clean up comments and move related declarations near
	each other.
	* par-prag.adb: Process Warning_As_Error. This is necessary
	because many style warning happen during parsing.
	* sem_prag.adb: Use new Acquire_Warning_Match_String.
	* sem_util.ads, sem_util.adb (Acquire_Warning_Match_String): New
	function shared by par-prag.adb and sem_prag.adb. Replaces the
	procedure in sem_prag.adb. Avoid use of global variables.
	* stringt.ads, stringt.adb (To_String): New function to convert
	String_Id to String.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Document the
	new feature.
	* gnat_rm.texi: Regenerate.

From-SVN: r274721