[Ada] More permissive use of GNAT attribute Enum_Rep

[Ada] More permissive use of GNAT attribute Enum_Rep

This patch allows the prefix of the attribute Enum_Rep to be an
attribute referece (such as Enum_Type'First). A recent patch had
restricted the prefix to be an object of a discrete type, which is
incompatible with orevious usage.

2019-07-01  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_attr.adb (Analyze_Attribute, case Enum_Rep): Allow prefix
	of attribute to be an attribute reference of a discrete type.

gcc/testsuite/

	* gnat.dg/enum_rep.adb, gnat.dg/enum_rep.ads: New testcase.

From-SVN: r272881