[Ada] Crash on derived type with rep. clause and renamed discriminant

[Pierre-Marie de Rodat]

This patch fixes a compiler abort on a declaration of subtype of a
derived type DT whose declaration renames a discriminant of its parent
type T, when there is a record representation clause for DT.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch5.adb (Find_Component): If the target type is a derived
	record type and the required component is a discriminant that is
	renamed in the derived type declaration, use the name of the
	original discriminant to locate the intended target component.
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