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

[Thomas Preud'homme via llvm-commits]

================
@@ -2486,6 +2489,14 @@ static bool ValidatePrefixes(StringRef Kind, StringSet<> &UniquePrefixes,
              << "check and comment prefixes: '" << Prefix << "'\n";
       return false;
     }
+    for (StringRef Directive : Directives) {
----------------
RoboTux wrote:

I think a prefix that looks like a prefix + directive is very misleading and can easily lead to bug as the test gets updated. E.g. a prefix CHECK-NEXT could easily go wrong. That sounds like a good change to me but should have a lit test indeed.

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