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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

Thank you for reviewing the patch!

> > from std::optionalstd::string
> 
> I assume you have some `<` and `>` here but they aren't getting rendered properly. Probably need backticks around the symbols.

Yeah, the github web interface displays these things in a funny way.  The actual commit message should be unaffected though.

https://github.com/llvm/llvm-project/pull/94655