[llvm] Fix file index verifier when there is no file name in the prologue. (PR #77004)

[Adrian Prantl via llvm-commits]

================
@@ -0,0 +1,356 @@
+# RUN: rm -rf %t && mkdir -p %t
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adrian-prantl wrote:

No need to create a directory:
```suggestion
 # RUN: yaml2obj %s -o %t.o
 # RUN: llvm-dwarfdump --debug-line --verify %t.o 2>&1 | FileCheck %s
```

https://github.com/llvm/llvm-project/pull/77004