[Ada] Enumeration types with non-standard representation

[Pierre-Marie de Rodat]

The compiler may report errors on enumeration types with non-standard
representation (i.e. at least one literal has a representation value
different from its 'Pos value) processing attribute 'Enum_Rep.

It may also generate wrong code for the evaluation of 'Enum_Rep raising
Constraint_Error at runtime.

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

2018-08-21  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* checks.ads (Determine_Range): Adding documentation.
	* checks.adb (Determine_Range): Don't deal with enumerated types
	with non-standard representation.
	(Convert_And_Check_Range): For conversion of enumeration types
	with non standard representation to an integer type perform a
	direct conversion to the target integer type.

gcc/testsuite/

	* gnat.dg/enum4.adb: New testcase.
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