[Ada] Ada 2020: Raise expressions in limited contexts (AI12-0172)

[Ada] Ada 2020: Raise expressions in limited contexts (AI12-0172)

This patch adds support for the use of raise expressions in more
limited contexts (as described in the Ada Isssue AI12-0172).

2019-09-17  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_ch3.adb (Build_Record_Init_Proc): Do not generate code to
	adjust the tag component when the record is initialized with a
	raise expression.
	* sem_aggr.adb (Valid_Limited_Ancestor): Return True for
	N_Raise_Expression nodes.
	(Valid_Ancestor_Type): Return True for raise expressions.
	* sem_ch3.adb (Analyze_Component_Declaration): Do not report an
	error when a component is initialized with a raise expression.
	* sem_ch4.adb (Analyze_Qualified_Expression): Do not report an
	error when the aggregate has a raise expression.

gcc/testsuite/

	* gnat.dg/limited4.adb: New testcase.

From-SVN: r275776