FindOpenMP: unable to detect OpenMP on windows with MSVC+LLVM

I have a simple CMakeLists.txt:

CMAKE_MINIMUM_REQUIRED(VERSION 3.6)

PROJECT(test_openmp CXX)
find_package(OpenMP)
message(">> ${OpenMP_FOUND} ${OpenMP_VERSION}")
add_executable(parallel parallel_region.cpp)

As indicated, I'd love to generate build files for some OpenMP code with cmake. The source code file looks like this:

#include <iostream>
#include "omp.h"
#include <unordered_map>

int main(int argc, char *argv[])
{
	
  int result = 0;
#pragma omp parallel
  {
    int ID = omp_get_thread_num();
#pragma omp critical
    {
      std::cout << "adding " << ID << "\n";
      result += ID;
    }
  }

  #ifdef _OPENMP
  std::unordered_map<unsigned, std::string> map{
	  {200505,"2.5"},{200805,"3.0"},{201107,"3.1"},{201307,"4.0"},{201511,"4.5"} };
  std::cout << int(_OPENMP) << " " << "(OpenMP " << map.at(_OPENMP).c_str() << ") ";
  #endif
  std::cout << " n_threads = " << omp_get_num_threads() << " result is " << result << "\n";
  return 0;
}

I am successful in running cmake with MSVC 15 2017:

$ cmake -Ax64 ..
Re-run cmake no build system arguments
-- Building for: Visual Studio 15 2017
-- The CXX compiler identification is MSVC 19.16.27030.1
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.16.27023/bin/Hostx86/x64/cl.exe
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.16.27023/bin/Hostx86/x64/cl.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found OpenMP_CXX: -openmp (found version "2.0")
-- Found OpenMP: TRUE (found version "2.0")
>> TRUE
-- Configuring done
-- Generating done
-- Build files have been written to: C:/Users/steinbac/development/cmake_sandbox/find_openmp_vs_llvm/build

My ultimate goal is to use openmp shipped with LLVM/clang 8.0.0 and higher. So, I installed llvm and ran cmake:

$ cmake -Tllvm -Ax64 ..
Re-run cmake no build system arguments
-- Building for: Visual Studio 15 2017
-- The CXX compiler identification is Clang 8.0.0 with MSVC-like command-line
-- Check for working CXX compiler: C:/Program Files/LLVM/bin/clang-cl.exe
-- Check for working CXX compiler: C:/Program Files/LLVM/bin/clang-cl.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)
-- Could NOT find OpenMP (missing: OpenMP_CXX_FOUND)
>> FALSE
-- Configuring done
-- Generating done
-- Build files have been written to: C:/Users/steinbac/development/cmake_sandbox/find_openmp_vs_llvm/build

The subsequent call to cmake --build . fails due to linker errors. Building the source code file by hand works:

$ clang++.exe -fopenmp parallel_region.cpp
parallel_region-75f09d.o : warning LNK4217: locally defined symbol __std_terminate imported in function "?dtor$6@?0?.omp_outlined.@4HA" (?dtor$6@?0?.omp_outlined.@4HA)
parallel_region-75f09d.o : warning LNK4217: locally defined symbol _CxxThrowException imported in function "public: void __cdecl std::ios_base::clear(int,bool)" (?clear@ios_base@std@@QEAAXH_N@Z)

The produced executable runs perfectly fine. I am wondering why cmake cannot set up the right flags for clang.