[Ada] Spurious discriminant check on "for of" loop

[Pierre-Marie de Rodat]

When using a "for ... of" loop with an element of a record type with
defaulted discriminants, a spurious disciminant check is emitted.

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

gcc/ada/

	* sem_ch8.adb (Check_Constrained_Object): Suppress discriminant
	checks when the type has default discriminants and comes from
	expansion of a "for of" loop.
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