Implement explicit conversions ops as specified in N2437.

	* decl.c (grokdeclarator): Handle explicit conversion ops.
	(check_initializer): Pass flags to store_init_value.
	* decl2.c (maybe_emit_vtables): Likewise.
	* init.c (expand_aggr_init_1): Likewise.
	* call.c (convert_class_to_reference): Take flags parm,
	check DECL_NONCONVERTING_P.
	(build_user_type_conversion_1): Check DECL_NONCONVERTING_P.
	(add_builtin_candidates): Simplify getting type of conversion.
	(build_object_call): Likewise.  Check DECL_NONCONVERTING_P.
	(implicit_conversion): Pass through LOOKUP_ONLYCONVERTING.
	(reference_binding): Take flags parm.  Direct-initialize copy parm.
	(add_function_candidate): Direct-initialize the copy parm.
	(add_conv_candidate): Use LOOKUP_IMPLICIT, not LOOKUP_NORMAL.
	(build_builtin_candidate): Add LOOKUP_ONLYCONVERTING.
	(conditional_conversion): Likewise.
	(convert_like_real): Only complain about DECL_NONCONVERTING_P
	constructors.
	(perform_implicit_conversion_flags): Add flags parm to
	perform_implicit_conversion.  Improve diagnostics.
	* cp-tree.h (LOOKUP_IMPLICIT): New macro.
	(LOOKUP_COPY_PARM): New bit macro.
	* cvt.c (build_expr_type_conversion): Check DECL_NONCONVERTING_P.
	* typeck.c (convert_for_assignment): Take flags parm, pass it to
	perform_implicit_conversion_flags.
	(cp_build_modify_expr): Pass flags to convert_for_assignment.
	(convert_for_initialization): Likewise.
	* typeck2.c (store_init_value): Take flags parm, pass to
	digest_init_flags.
	(digest_init_flags): Add flags parm to digest_init.
	(digest_init_r): Take flags parm, pass to convert_for_initialization.
	(process_init_constructor_array): Pass it.
	(process_init_constructor_record): Likewise.
	(process_init_constructor_union): Likewise.

From-SVN: r147677