[Ada] Crash on 'Img attribute

[Pierre-Marie de Rodat]

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.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.
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