[Ada] Simplify getting discriminant value from a list of constraints

[Pierre-Marie de Rodat]

Routine Get_Discr_Value takes a name of a discriminant, not its entity,
so its parameter should be a Node_Id, not an Entity_Id. Also, fix typo
in its comment and refactor repeated calls in the body. Semantics is
unaffected.

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

2020-06-12  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_ch3.adb (Get_Discr_Value): Cleanup.
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