compiletest: "fix" FileCheck with --allow-unused-prefixes

The default of --allow-unused-prefixes used to be false, but in LLVM
change 87dbdd2 (https://reviews.llvm.org/D95849) the default became
true. I'm not quite sure how we could do better here (specifically not
providing the CHECK prefix when it's not needed), but this seems to work
for now.

Author: durin42

src/tools/compiletest/src/runtest.rs

@@ -2293,7 +2293,7 @@ impl<'test> TestCx<'test> {
         // For now, though…
         if let Some(rev) = self.revision {
             let prefixes = format!("CHECK,{}", rev);
-            filecheck.args(&["--check-prefixes", &prefixes]);
+            filecheck.args(&["--allow-unused-prefixes", "--check-prefixes", &prefixes]);
         }
         self.compose_and_run(filecheck, "", None, None)
     }