[ProfData] Improve efficiency of reader (#169730)
Pre-reserve space in the map before inserting. In release builds, 9.4%
of all CPU time is spent in llvm::sampleprof::ProfileSymbolList::add. Of
that 9.4%, roughly half is in llvm::DenseMapBase::grow.
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Co-authored-by: mxms <mxms@google.com>
Co-authored-by: Vitaly Buka <vitalybuka@gmail.com>
