[Ada] Fix compiler abort on invalid discriminant constraint

[Ada] Fix compiler abort on invalid discriminant constraint

This patch fixes a compiler abort on a discriminant constraint when the
constraint is a subtype indication.

2018-05-22  Patrick Bernardi  <bernardi@adacore.com>

gcc/ada/

	* sem_ch3.adb (Build_Discriminant_Constraints): Raise an error if the
	user tries to use a subtype indication as a discriminant constraint.

gcc/testsuite/

	* gnat.dg/discr50.adb: New testcase.

From-SVN: r260525