[llvm] [Windows][test] Fix "LLVM" test failures when LLVM_WINDOWS_PREFER_FORWARD_SLASH is ON (PR #184556)

[Junji Watanabe via llvm-commits]

Jwata wrote:

> Thinking more about the llvm-objdump and llvm-cov tests: shouldn't these tests show that the correct slash is used, based on the value of LLVM_WINDOWS_PREFER_FORWARD_SLASH?

We need to switch the expectations between `\` and `/` based on LLVM_WINDOWS_PREFER_FORWARD_SLASH: 

without LLVM_WINDOWS_PREFER_FORWARD_SLASH=on
```
// TEXT-INDEX: \tmp\native_separators.c
...
```

with LLVM_WINDOWS_PREFER_FORWARD_SLASH=on
```
// TEXT-INDEX: /tmp/native_separators.c
...
```

So, I would  add another test case something like `llvm-cov/native_separators_preffer_forward_slash.c`.

Does that make sense?

https://github.com/llvm/llvm-project/pull/184556