ast: Use std::optional in CodegenLLVM::CodegenLLVM call

[kraj]

Fixes build with clang-16

src/ast/passes/codegen_llvm.cpp:63:53: error: use of undeclared identifier 'Optional'; did you mean 'std::optional'?

Checklist

• Language changes are updated in man/adoc/bpftrace.adoc and if needed in docs/reference_guide.md
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests

[viktormalik]

Thanks!