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

[Kazu Hirata via llvm-commits]

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@@ -552,6 +552,41 @@ static Error writeMemProf(
               memprof::MaximumSupportedVersion));
 }
 
+size_t InstrProfWriter::writeHeader(IndexedInstrProf::Header &Header,
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kazutakahirata wrote:

May I suggest `const IndexedInstrProf::Header &`?

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