AstGen: disallow '-0' integer literal

The intent here is ambiguous: this resolves to the comptime_int '0', but
it's likely the user meant to use a floating-point literal.

Resolves: #16890

test/cases/compile_errors/negative_zero_literal.zig

@@ -0,0 +1,11 @@
+export fn foo() void {
+    _ = -0;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:10: error: integer literal '-0' is ambiguous
+// :2:10: note: use '0' for an integer zero
+// :2:10: note: use '-0.0' for a floating-point signed zero