Fix rejection of arrays with eight or nine elements. (#3199)

In general, LLVM's parsing of decimal integers to `APInt`s forms an
`APInt` that is 4n bits wide, where n is the length of the integer, and
the `isNegative` check only checks the high bit.

In this case, we form an `APInt` that is four bits wide, with the high
bit set, which we reject because we think it's "negative". These two
array lengths are the only ones where this happens -- if the decimal
integer value is two characters long, we form an `APInt` that is eight
bits wide but holds a value < 100, so the high bit is never set, and the
same applies for longer integers too.

An `IntegerLiteral` is never negative, so we don't need the `isNegative`
check, and in fact it only detects the n=8 and n=9 cases.

toolchain/check/handle_array.cpp

@@ -37,7 +37,8 @@ auto HandleArrayExpression(Context& context, Parse::Node parse_node) -> bool {
   if (bound_node.kind() == SemIR::NodeKind::IntegerLiteral) {
     auto bound_value = context.semantics_ir().GetIntegerLiteral(
         bound_node.GetAsIntegerLiteral());
-    if (!bound_value.isNegative() && bound_value.getBitWidth() <= 64) {
+    // TODO: Produce an error if the array type is too large.
+    if (bound_value.getBitWidth() <= 64) {
       context.AddNodeAndPush(
           parse_node,
           SemIR::Node::ArrayType::Make(