[llvm] [lit] Add --report-failures-only option for lit test reports (PR #115439)

[James Henderson via llvm-commits]

================
@@ -0,0 +1,17 @@
+# UNSUPPORTED: system-windows
+
+# Check xunit output
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jh7370 wrote:

```suggestion
## Check xunit output.
```
A practice I'm trying to encourage with newer tests is to use double comment markers to help actual comments stand out from lit and FileCheck directives.

Also missing trailing full stop.

https://github.com/llvm/llvm-project/pull/115439