[llvm] [nfc][InstrFDO]Encapsulate header writes in a class member function (PR #90142)

[Kazu Hirata via llvm-commits]

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@@ -204,6 +204,12 @@ class InstrProfWriter {
   void addTemporalProfileTrace(TemporalProfTraceTy Trace);
 
   Error writeImpl(ProfOStream &OS);
+
+  // Writes known header fields and preserves space for fields whose value are
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kazutakahirata wrote:

```suggestion
  // Writes known header fields and reserves space for fields whose value are
```

https://github.com/llvm/llvm-project/pull/90142