[Bug]: variant index returns wrong value

### Describe the issue

variant index returns wrong value

### Steps to reproduce the problem

std::variant test_std = "test";
absl::variant test_absl = "test";
std::cout << test_std.index() << " <->" << test_absl.index(); // 0 <-> 1

### What version of Abseil are you using?

Current Chromium Code

### What operating system and version are you using?

Win 11

### What compiler and version are you using?

Current Chromium Code

### What build system are you using?

Current Chromium Code

### Additional context

_No response_