[CMake] Reinstate LLDB_CAN_USE_LLDB_SERVER

We cannot manipulate the LLDB_TOOL_LLDB_SERVER_BUILD directly from
LLDBConfig.cmake because this would set the variable before the option
is defined in AddLLVM.cmake. Instead, we need to use the
LLDB_CAN_USE_LLDB_SERVER variable to conditionally add the lldb-server
subdirectory. This should ensure the variable doesn't get cleared.

llvm-svn: 360631