[nfc][InstrProf]Remove 'offsetOf' when parsing indexed profiles (#93346)

- In `Header::readFromBuffer`, read the buffer in the forward direction by using `readNext`.
- When compute the header size, spell out the constant.

With the changes above, we can remove `offsetOf` in InstrProf.cpp

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Co-authored-by: Kazu Hirata <kazu@google.com>