[Ada] Crash on 'Img attribute

[Ada] Crash on 'Img attribute

This patch fixes and issue whereby applying 'Img to a constant
enumerated character type would result in a compiler crash when
assertions are enabled and infinite recursion when they are not.

2019-07-09  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_eval.adb (Expr_Value_E): Add conditional to correctly
	handle constant enumerated character types.

gcc/testsuite/

	* gnat.dg/image1.adb: New testcase.

From-SVN: r273292