[lldb-dap] Addressing ubsan enum usage. (#133542)
Running tests with ubsan enabled showed that the current
`protocol::AdapterFeature` and `protocol::ClientFeature` enums are
incorrectly defined if we want to use them in a `llvm::DenseSet`. The
enums are currently untyped and this results in the undefined behavior.
Adding `FLAGS_ENUM()` wrappers to all the enums in the
`lldb-dap::protocol` namespace to ensure they're typed so they can be
used with types like `llvm::DenseSet`.
