[lldb] [lldb-server] Refactor ConnectToRemote()

Refactor ConnectToRemote() to improve readability and make future
changes easier:

1. Replace static buffers with std::string.
2. When handling errors, prefer reporting the actual error over dumb
   'connection status is not success'.
3. Move host/port parsing directly into reverse_connection condition
   that is its only user, and simplify it to make its purpose (verifying
   that a valid port is provided) clear.
4. Use llvm::errs() and llvm::outs() instead of fprintf().

Differential Revision: https://reviews.llvm.org/D11196