[memprof] Use std::vector<Frame> instead of llvm::SmallVector<Frame> (NFC) (#94432)

This patch replaces llvm::SmallVector<Frame> with std::vector<Frame>.

llvm::SmallVector<Frame> sets aside one inline element.  Meanwhile,
when I sort all call stacks by their lengths, the length at the first
percentile is already 2.  That is, 99 percent of call stacks do not
take advantage of the inline element.

Using std::vector<Frame> reduces the cycle and instruction counts by
11% and 22%, respectively, with "llvm-profdata show" modified to
deserialize all MemProfRecords.