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

[James Henderson via llvm-commits]

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@@ -1,2 +1,5 @@
 if not "X86" in config.root.targets:
     config.unsupported = True
+
+if "windows-prefer-forward-slash" in config.available_features:
+    lit_config.params["use_normalized_slashes"] = "1"
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jh7370 wrote:

What's the impact of this? Do we still need it?

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