[Ada] Add second warning for convention C_Variadic_n

[Pierre-Marie de Rodat]

This adds a second warning related to the new C_Variadic_n convention,
for the cases where the aspect/pragma is applied to a subprogram with
exactly n parameters since, in this case, the aspect/pragma is useless.

The warning is given as such:

btest.ads:16:05: warning: subprogram should have at least 4 parameters

for an aspect/pragma C_Variadic_3.

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

2020-06-18  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_prag.adb (Process_Convention): Give a warning on C_Variadic_n
	being applied to a subprogram with exactly n parameters.
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