Constant evaluation rejects "new wchar_t[n]{L\"...\"}" although n is large enough #501

During constant evaluation, the check that the initializer of an array
new-expression fits in the allocated array uses the size in bytes of a string
literal initializer instead of its number of characters. So when the elements
are wider than one byte, an array that is large enough is reported as too
small:

constexpr int f(int n) {
  auto p = new wchar_t[n]{L"ab"};
  int r = p[1];
  delete[] p;
  return r;
}
static_assert(f(3) == L'b');

error: expression must have a constant value
note: number of dynamically-allocated elements (3) too small for initializer

The same happens with char16_t and char32_t arrays; char and char8_t
arrays are not affected. (Found by CodeRabbit while reviewing
#495.)