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

[Snehasish Kumar via llvm-commits]

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@@ -197,13 +197,26 @@ class InstrProfWriter {
                      const OverlapFuncFilters &FuncFilter);
 
 private:
+  struct HeaderFieldOffsets {
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snehasish wrote:

Add a comment describing what this struct holds and how the contents are used.

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