[Ada] Mimic the C++ ABI when passing class-wide conversion actuals

[Pierre-Marie de Rodat]

This patch does not affect the behavior of Ada-only code but improves
consistency with the code generated by the C++ compiler.

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

2018-09-26  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Actuals): If the formal is a class-wide
	type conversion then do not skip resolving and expanding the
	conversion; required to displace the pointer to the object and
	reference the dispatch table associated with the target
	interface type.
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