[Ada] Simplify building of entity occurrences

[Pierre-Marie de Rodat]

Code cleanup related to expansion of predefined equality for GNATprove;
semantics is unaffected.

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

gcc/ada/

	* tbuild.adb (New_Occurrence_Of): Simplify by reusing
	Make_Identifier.
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