[lldb] Set CMAKE_CXX_STANDARD before including LLDBStandalone

Set the C++ language standard before including LLDBStandalone.cmake.
Otherwise we risk building some of our dependencies (such as llvm_gtest)
without C++ 17 support.

This should fix the standalone bot [1] which is currently failing with the
following error:

  test-port.h:841:12: error: no member named 'tuple' in namespace 'std'
  using std::tuple;

[1] https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake-standalone