[Ada] Fix internal error on extension of record with representation clause

[Ada] Fix internal error on extension of record with representation clause

This fixes a long-standing issue present for extensions of tagged record
types with a representation clause: the clause is correctly inherited
for components inherited in the extension but the position and size are
not, which fools the logic of Is_Possibly_Unaligned_Object.

This can result in an attempt to take the address of a component not
aligned on a byte boundary, which is then flagged as an internal error.

2018-08-21  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_util.adb (Is_Possibly_Unaligned_Object): For the case of a
	selected component inherited in a record extension and subject
	to a representation clause, retrieve the position and size from
	the original record component.

gcc/testsuite/

	* gnat.dg/rep_clause7.adb: New testcase.

From-SVN: r263717