[Ada] Spurious error in discriminated aggregate

[Ada] Spurious error in discriminated aggregate

This patch fixes a bug in which a spurious error is given on an
aggregate of a type derived from a subtype with a constrained
discriminant.

2019-08-14  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_aggr.adb (Init_Hidden_Discriminants): Avoid processing the
	wrong discriminant, which could be of the wrong type.

gcc/testsuite/

	* gnat.dg/discr57.adb: New testcase.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@274458 138bc75d-0d04-0410-961f-82ee72b054a4