[Ada] Warn about overlapping actuals in all versions of Ada

[Pierre-Marie de Rodat]

Overlapping actuals of elementary types are illegal since Ada 2012; all
other overlapping actuals are now reported as warnings regardless of the
Ada version (because they are equally suspicious).

We even had a comment:

  "Overlap is only illegal in Ada 2012 in the case of elementary
  types (passed by copy). For other types we always have a warning in
  all versions."

but it was not really implemented until now.

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

gcc/ada/

	* sem_warn.adb (Warn_On_Overlapping_Actuals): Cleanup conditions
	related to Ada_Version.
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