[Ada] Do not treat pragma Compile_Time_Warning as error even with -gnatwe

[Ada] Do not treat pragma Compile_Time_Warning as error even with -gnatwe

2020-06-18  Ghjuvan Lacambre  <lacambre@adacore.com>

gcc/ada/

	* erroutc.ads: Declare new Is_Compile_Time_Msg boolean,
	add new Compile_Time_Pragma field to Error_Msg_Object type.
	(Count_Compile_Time_Pragma_Warnings): New function.
	* erroutc.adb (Count_Compile_Time_Pragma_Warnings): New
	function.
	(Compilation_Errors): Take Compile_Time warnings into account
	when tallying Errors/Warnings.
	* errout.ads (Error_Msg): New procedure.
	* errout.adb (Error_Msg): New procedure.
	(Error_Msg_Internal): Set new Compile_Time_Pragma field in
	Error_Msg_Object.
	* errutil.adb (Error_Msg): Set new Compile_Time_Pragma field in
	Error_Msg_Object.
	* sem_prag.adb (Validate_Compile_Time_Warning_Or_Error): Pass
	True to new Error_Msg procedure.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Update doc for
	the Compile_Time_Warning pragma.
	* gnat_rm.texi: Regenerate.
	* opt.ads: Update comment.