re PR debug/58150 (debug info about definition of enum class not emitted if the declaration was already used in a class)

	PR debug/58150
	* dwarf2out.c (gen_enumeration_type_die): Don't guard adding
	DW_AT_declaration for ENUM_IS_OPAQUE on -gdwarf-4 or -gno-strict-dwarf,
	but on TYPE_SIZE.  Don't do anything for ENUM_IS_OPAQUE if not creating
	a new die.  Don't set TREE_ASM_WRITTEN if ENUM_IS_OPAQUE.  Guard
	addition of most attributes on !orig_type_die or the attribute not
	being present already.  Assert TYPE_VALUES is NULL for ENUM_IS_OPAQUE.

	* g++.dg/debug/dwarf2/enum2.C: New test.

From-SVN: r258434