[Ada] Preparation for new description of interface thunks

[Ada] Preparation for new description of interface thunks

This adjusts and exposes a couple of functions of the front-end used for the
generation of interface thunks so as to make them callable from gigi.  This
also propagates the debug info setting from the targets to the thunks so as
to make stepping into primitives work better in the debugger.

2018-09-26  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_disp.adb (Expand_Interface_Conversion): Use Present test.
	(Expand_Interface_Thunk): Propagate debug info setting from
	target.
	* exp_util.ads (Find_Interface_Tag): Adjust comment.
	* exp_util.adb (Find_Interface_Tag): Remove assertions of
	success.
	* sem_util.adb (Is_Variable_Size_Record): Only look at
	components and robustify the implementation.
	* fe.h (Find_Interface_Tag): Declare.
	(Is_Variable_Size_Record): Likewise.

From-SVN: r264614