[Ada] Crash on Indefinite_Hashed_Maps with -gnata -gnateV

[Ada] Crash on Indefinite_Hashed_Maps with -gnata -gnateV

This patch corrects the generation of helper functions which verify the
validity of record type scalar discriminants and scalar components when
switches -gnata (assertions enabled) and -gnateV (validity checks on
subprogram parameters) are in effect.

2018-07-16  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* exp_attr.adb (Build_Record_VS_Func): Handle corner cases dealing with
	class-wide types and record extensions.

gcc/testsuite/

	* gnat.dg/validity_check3.adb, gnat.dg/validity_check3.ads: New
	testcase.

From-SVN: r262715