This patch implements P0388R4, Permit conversions to arrays of unknown bound,
<http://wg21.link/p0388r4>.  CWG 393 allowed references to arrays of unknown
bound and this C++20 feature allows conversions like

  void f(int(&)[]);
  int arr[1];

  void g() { f(arr); }
  int(&r)[] = arr;

The proposal seemed fairly straightforward but it turned out to be quite
shifty.  I found out that I needed to implement DR 2352 (done), and also
DR 1307 (done in this patch).  The latter DR added wording for
list-initialization ranking of references to arrays which this proposal
extends.  DR 330 was also implemented in this patch.

	PR c++/91364 - P0388R4: Permit conversions to arrays of	unknown bound.
	PR c++/69531 - DR 1307: Differently bounded array parameters.
	PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers.
	* call.c (build_array_conv): Build ck_identity at the beginning
	of the conversion.
	(standard_conversion): Pass bounds_none to comp_ptr_ttypes_const.
	(maybe_warn_array_conv): New.
	(convert_like_real): Call it.  Add an error message about converting
	from arrays of unknown bounds.
	(conv_get_original_expr): New.
	(nelts_initialized_by_list_init): New.
	(conv_binds_to_array_of_unknown_bound): New.
	(compare_ics): Implement list-initialization ranking based on
	array sizes, as specified in DR 1307 and P0388R.
	* cp-tree.h (comp_ptr_ttypes_const): Adjust declaration.
	(compare_bounds_t): New enum.
	* typeck.c (comp_array_types): New bool and compare_bounds_t
	parameters.  Use them.
	(structural_comptypes): Adjust the call to comp_array_types.
	(similar_type_p): Handle ARRAY_TYPE.
	(build_const_cast_1): Pass bounds_none to comp_ptr_ttypes_const.
	(comp_ptr_ttypes_real): Don't check cv-quals of ARRAY_TYPEs.  Use
	comp_array_types to compare array types.  Look through arrays as per
	DR 330.
	(comp_ptr_ttypes_const): Use comp_array_types to compare array types.
	Look through arrays as per DR 330.

	* g++.dg/conversion/qual1.C: New test.
	* g++.dg/conversion/qual2.C: New test.
	* g++.dg/conversion/qual3.C: New test.
	* g++.dg/conversion/ref2.C: New test.
	* g++.dg/conversion/ref3.C: New test.
	* g++.dg/cpp0x/initlist-array3.C: Remove dg-error.
	* g++.dg/cpp0x/initlist-array7.C: New test.
	* g++.dg/cpp0x/initlist-array8.C: New test.
	* g++.dg/cpp2a/array-conv1.C: New test.
	* g++.dg/cpp2a/array-conv10.C: New test.
	* g++.dg/cpp2a/array-conv11.C: New test.
	* g++.dg/cpp2a/array-conv12.C: New test.
	* g++.dg/cpp2a/array-conv13.C: New test.
	* g++.dg/cpp2a/array-conv14.C: New test.
	* g++.dg/cpp2a/array-conv15.C: New test.
	* g++.dg/cpp2a/array-conv16.C: New test.
	* g++.dg/cpp2a/array-conv17.C: New test.
	* g++.dg/cpp2a/array-conv2.C: New test.
	* g++.dg/cpp2a/array-conv3.C: New test.
	* g++.dg/cpp2a/array-conv4.C: New test.
	* g++.dg/cpp2a/array-conv5.C: New test.
	* g++.dg/cpp2a/array-conv6.C: New test.
	* g++.dg/cpp2a/array-conv7.C: New test.
	* g++.dg/cpp2a/array-conv8.C: New test.
	* g++.dg/cpp2a/array-conv9.C: New test.
	* g++.old-deja/g++.bugs/900321_01.C: Adjust dg-error.

	* testsuite/23_containers/span/lwg3255.cc: Adjust test to match the
	post-P0388R4 behavior.

From-SVN: r276771