[Ada] Missing error on illegal access to discriminant

[Ada] Missing error on illegal access to discriminant

The compiler does not report an error on the illegal access to a renamed
discriminant when the actual object is a parameter of a subprogram.

2018-05-24  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_ch3.adb (Is_Visible_Component): For untagged types add missing
	check for renamed discriminants.
	* sem_ch4.adb (Analyze_Overloaded_Selected_Component,
	Analyze_Selected_Component, Check_Misspelled_Selector): For calls to
	Is_Visible_Component pass the associated selector node to allow
	checking renamed discriminants on untagged types.

gcc/testsuite/

	* gnat.dg/discr52.adb: New testcase.

From-SVN: r260664