[Ada] Wrong dispatching call in type with aspect Implicit_Dereference

[Pierre-Marie de Rodat]

When a record type with an an access to class-wide type discriminant
has aspect Implicit_Dereference, and the discriminant is used as the
controlling argument of a dispatching call, the compiler may generate
wrong code to dispatch the call.

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

2019-08-13  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Selected_Component): When the type of the
	component is an access to a class-wide type and the type of the
	context is an access to a tagged type the relevant type is that
	of the component (since in such case we may need to generate
	implicit type conversions or dispatching calls).

gcc/testsuite/

	* gnat.dg/tagged3.adb, gnat.dg/tagged3_pkg.adb,
	gnat.dg/tagged3_pkg.ads: New testcase.
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