fix: clover/llvm: replace llvm::None with std::nullopt

llvm::None was deprecated and builds started failing with

error: ‘None’ is not a member of ‘llvm’

Instead of using the temporarily available include in ADT which would add a deprecation warning to the build, directly replace llvm::None with the recommended std::nullopt.

Reference: https://github.com/llvm/llvm-project/blob/d4f38ef288c3a4cf2318182c8585a5c7e760877a/llvm/include/llvm/ADT/None.h