[Ada] Wrong accessibility level under -gnat12

[Ada] Wrong accessibility level under -gnat12

For an anonymous allocator whose type is that of a stand-alone object of
an anonymous access-to-object type, the accessibility level is that of
the declaration of the stand-alone object; however it was incorrectly
computed as being library level compiling under -gnat12 mode.

2019-07-05  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_N_Type_Conversion): Do not apply an
	accessibility check when the conversion is an access to
	class-wide interface type and it is an actual parameter.
	* exp_ch6.adb (Expand_Call_Helper): Add documentation on the
	accessibility level of an anonymous allocator defining the value
	of an access parameter.
	* sem_util.ads, sem_util.adb (Dynamic_Accessibility_Level): Add
	support for an anonymous allocator whose type is that of a
	stand-alone object of an anonymous access to object type.

gcc/testsuite/

	* gnat.dg/access6.adb: New testcase.

From-SVN: r273115