[Ada] Refine type of Get_Homonym_Number result

[Pierre-Marie de Rodat]

Routine Get_Homonym_Number always returns a positive number. This is
explained in its comment and is evident from its body. No test attached,
because semantics is unaffected.

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

2019-09-18  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* exp_dbug.ads, exp_dbug.adb (Get_Homonym_Number): Refine type
	from Nat to Pos.
	* sem_util.adb (Add_Homonym_Suffix): Refine type of a local
	variable.
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