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

[Kazu Hirata via llvm-commits]

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@@ -552,6 +552,29 @@ static Error writeMemProf(
               memprof::MaximumSupportedVersion));
 }
 
+uint64_t InstrProfWriter::writeHeader(const IndexedInstrProf::Header &Header,
+                                      const bool WritePrevVersion,
+                                      ProfOStream &OS) {
+  // Records the offset before writing any fields.
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kazutakahirata wrote:

Use the infinitive form to be consistent with `Reserve the space ...` below.

```suggestion
  // Record the offset before writing any fields.
```

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