[llvm] [FileCheck] improve prefix validation (PR #92248)

[via llvm-commits]

================
@@ -1,5 +1,5 @@
-# Comment prefixes plus check directive suffixes are not comment directives
-# and are treated as plain text.
+# Comment prefixes plus check directive suffixes are forbidden.
----------------
klensy wrote:

There still some unresolved error, working on it
```
# note: command had no output on stdout or stderr
# error: command failed with exit status: 2

```

https://github.com/llvm/llvm-project/pull/92248