[LLVM] Use std::nullopt instead of llvm::None

[kparzysz-quic]

Pass std::nullopt to initialization of PassBuilder for PGOOptions. LLVM is moving away from its own Optional type to std::optional.

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[Icemist]

LGTM. Looks backward compatible because llvm::Optional<T> has a constructor from std::nullopt_t.

It is about this change in the main branch:
LLVM::Optional<PGOOptions> PGOOpt = LLVM::None,
=>
std::optional<PGOOptions> PGOOpt = std::nullopt,