Implement P0127R2, Declaring non-type parameters with auto.

Implement P0127R2, Declaring non-type parameters with auto.

gcc/cp/
	* cp-tree.h (enum auto_deduction_context): Add adc_unify.
	* decl.c (grokdeclarator): Allow 'auto' in C++17 template non-type
	parameter types.
	* pt.c (do_auto_deduction): Add outer_targs parameter.
	(convert_template_argument): Call do_auto_deduction.  If adc_unify,
	don't give up on dependent init.
	(unify): Likewise.  In C++17, walk into the type of a
	TEMPLATE_PARM_INDEX.
	(for_each_template_parm): Add any_fn parameter.
	(struct pair_fn_data): Likewise.
	(for_each_template_parm_r): Call it for any tree.  In C++17, walk
	into the type of a TEMPLATE_PARM_INDEX.
	(zero_r, array_deduction_r, try_array_deduction): New.
	(type_unification_real): Call try_array_deduction.
	(get_partial_spec_bindings): Likewise.
gcc/c-family/
	* c-cppbuiltin.c (c_cpp_builtins): Define __cpp_template_auto.

From-SVN: r242017