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

[Fangrui Song via llvm-commits]

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

> Has it caused issues?

@klensy reported that
https://github.com/llvm/llvm-project/blame/main/lld/test/ELF/arm-exidx-shared.s used `// RUN: llvm-objdump -s --triple=armv7a-none-linux-gnueabi %t2 | FileCheck --check-prefix=CHECK-EXTAB-NEXT %s`.

`CHECK-EXTAB-NEXT` was very confusing. I fixed it.

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