[memprof] Use std::unique_ptr instead of std::optional (#94655)

Changing the type of Frame::SymbolName from std::optional<std::string>
to std::unique<std::string> reduces sizeof(Frame) from 64 to 32.

The smaller type reduces the cycle and instruction counts by 23% and
4.4%, respectively, with "llvm-profdata show" modified to deserialize
all MemProfRecords in a MemProf V2 profile.  The peak memory usage is
cut down nearly by half.