[Ada] Issue warning or error message on ignored typing constraint

[Ada] Issue warning or error message on ignored typing constraint

GNAT ignores the discriminant constraint on a component when it applies
to the type of the record being analyzed. Now issue a warning on Ada
code when ignoring this constraint, or an error on SPARK code.

2019-07-22  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_ch3.adb (Constrain_Access): Issue a message about ignored
	constraint.

gcc/testsuite/

	* gnat.dg/warn24.adb: New testcase.

From-SVN: r273684