c++: Implement P1009: Array size deduction in new-expressions.

c++: Implement P1009: Array size deduction in new-expressions.

This patch implements C++20 P1009, allowing code like

  new double[]{1,2,3}; // array bound will be deduced

Since this proposal makes the initialization rules more consistent, it is
applied to all previous versions of C++ (thus, effectively, all the way back
to C++11).

My patch is based on Jason's patch that handled the basic case.  I've
extended it to work with ()-init and also the string literal case.
Further testing revealed that to handle stuff like

  new int[]{t...};

in a template, we have to consider such a NEW_EXPR type-dependent.
Obviously, we first have to expand the pack to be able to deduce the
number of elements in the array.

Curiously, while implementing this proposal, I noticed that we fail
to accept

  new char[4]{"abc"};

so I've assigned 77841 to self.  I think the fix will depend on the
build_new_1 hunk in this patch.

The new tree.c function build_constructor_from_vec helps us morph
a vector into a CONSTRUCTOR more efficiently.

gcc/cp/ChangeLog:

	PR c++/93529
	* call.c (build_new_method_call_1): Use build_constructor_from_vec
	instead of build_tree_list_vec + build_constructor_from_list.
	* init.c (build_new_1): Handle new char[]{"foo"}.  Use
	build_constructor_from_vec instead of build_tree_list_vec +
	build_constructor_from_list.
	(build_new): Deduce the array size in new-expression if not
	present.  Handle ()-init.  Handle initializing an array from
	a string literal.
	* parser.c (cp_parser_new_type_id): Leave [] alone.
	(cp_parser_direct_new_declarator): Allow [].
	* pt.c (type_dependent_expression_p): In a NEW_EXPR, consider
	array types whose dimension has to be deduced type-dependent.

gcc/ChangeLog:

	PR c++/93529
	* tree.c (build_constructor_from_vec): New.
	* tree.h (build_constructor_from_vec): Declare.

gcc/testsuite/ChangeLog:

	PR c++/93529
	* g++.dg/cpp0x/sfinae4.C: Adjust expected result after P1009.
	* g++.dg/cpp2a/new-array1.C: New test.
	* g++.dg/cpp2a/new-array2.C: New test.
	* g++.dg/cpp2a/new-array3.C: New test.
	* g++.dg/cpp2a/new-array4.C: New test.

Co-authored-by: Jason Merrill <jason@redhat.com>