[Ada] Crash on Image and Value attributes

[Ada] Crash on Image and Value attributes

This patch fixes an issue whereby the creation of an enumeration within
package where Default_Scalar_Storage_Order is in effect may lead to a
crash when the attributes Image or Value are applied to objects of said
type or the type directly.

2019-07-08  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_imgv.adb (Build_Enumeration_Image_Tables): Default SSO for
	the building of image tables.
	(Expand_Image_Attribute): Minor cleanup.

gcc/testsuite/

	* gnat.dg/sso16.adb: New testcase.

From-SVN: r273199