[Ada] Missing accessibility check on discrim assignment

[Ada] Missing accessibility check on discrim assignment

This patch fixes an issue whereby assignments from anonymous access
descriminants which are part of stand alone objects of anonymous access
did not have runtime checks generated based on the accessibility level
of the object according to ARM 3.10.2 (12.5/3).

2019-09-18  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_N_Type_Conversion): Add calculation of an
	alternative operand for the purposes of generating accessibility
	checks.

gcc/testsuite/

	* gnat.dg/access8.adb, gnat.dg/access8_pkg.adb,
	gnat.dg/access8_pkg.ads: New testcase.

From-SVN: r275860