[Ada] Spurious error on interface conversion under ZFP

[Ada] Spurious error on interface conversion under ZFP

The frontend reports an error under ZFP when performing the type
conversion of a tagged object to one of its covered interface types.

2018-09-26  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_disp.adb (Expand_Interface_Conversion): No displacement of
	the pointer needed when the type of the operand is an interface
	type that maches the target type and we are compiling under
	configurable runtime. Adding also documentation explaining why
	this cannot be done when compiling with the full runtime.
	* exp_intr.adb: Update comment.

gcc/testsuite/

	* gnat.dg/interface8.adb, gnat.dg/interface8.ads: New testcase.

From-SVN: r264628