[Ada] Spurious error on precondition of subprogram instantiation

[Ada] Spurious error on precondition of subprogram instantiation

2019-12-13  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_ch13.adb (Analyze_Aspect_Specification): Processing a
	precondition, if the expression is of the form A and then B, do
	not generate separate Pre/Post aspects for the separate clases
	when we are processing a generic unit. Required to avoid
	reporting spurious visibility errors on the instantiations.

From-SVN: r279365