[Ada] Get rid of spurious error for _Tag on extension with reverse bit order

[Ada] Get rid of spurious error for _Tag on extension with reverse bit order

2019-10-10  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch13.adb (Adjust_Record_For_Reverse_Bit_Order): Do not use
	the Esize of the component to compute its layout, but only the
	Component_Clause.  Do not issue a warning for the _Tag
	component.  Also set the Esize of the component at the end of
	the layout.
	(Analyze_Record_Representation_Clause): Remove Hbit local
	variable.  Lay out the Original_Record_Component only if it's
	distinct from the component.
	(Check_Record_Representation_Clause): Fix off-by-one bug for the
	Last_Bit of the artificial clause built for the _Tag component.

From-SVN: r276827