c++: Fix array new with value-initialization [PR97523]

Since my r11-3092 the following is rejected with -std=c++20:

  struct T { explicit T(); };
  void fn(int n) {
    new T[1]();
  }

with "would use explicit constructor 'T::T()'".  It is because since
that change we go into the P1009 block in build_new (array_p is false,
but nelts is non-null and we're in C++20).  Since we only have (), we
build a {} and continue to build_new_1, which then calls build_vec_init
and then we error because the {} isn't CONSTRUCTOR_IS_DIRECT_INIT.

For (), which is value-initializing, we want to do what we were doing
before: pass empty init and let build_value_init take care of it.

For various reasons I wanted to dig a little bit deeper into this,
and as a result, I'm adding a test for [expr.new]/24 (and checked that
out current behavior matches clang++).

gcc/cp/ChangeLog:

	PR c++/97523
	* init.c (build_new): When value-initializing an array new,
	leave the INIT as an empty vector.

gcc/testsuite/ChangeLog:

	PR c++/97523
	* g++.dg/expr/anew5.C: New test.
	* g++.dg/expr/anew6.C: New test.