Implement P0138R2, C++17 construction rules for enum class values

Implement P0138R2, C++17 construction rules for enum class values

	Implement P0138R2, C++17 construction rules for enum class values
	* cp-tree.h (is_direct_enum_init): Declare.
	* decl.c (is_direct_enum_init): New function.
	(reshape_init): Use it.
	* typeck.c (convert_for_assignment): Likewise.

	* g++.dg/cpp1z/direct-enum-init1.C: New test.

From-SVN: r240449