CMake: make LLVM_OPTIMIZED_TABLEGEN friendly with LLVM_EXTERNAL_CLANG_SOURCE_DIR

[mehdi_amini]

Summary

This is need because of clang-tblgen

[beanz]

You should only set this if the variable is set, otherwise it will set the variable to empty and CMake's bad caching behavior will leave you stuck with that.

[mehdi_amini]

Only pass the option if it is set

[beanz]

LGTM

llvm/cmake/modules/CrossCompile.cmake

if(NOT IS_DIRECTORY ${LLVM_${target_name}_BUILD})
     message(STATUS "Configuring ${target_name} targets...")
     if (buildtype)
       set(build_type_flags "-DCMAKE_BUILD_TYPE=${buildtype}")
     endif()
     execute_process(COMMAND ${CMAKE_COMMAND} ${build_type_flags}
         -G "${CMAKE_GENERATOR}" -DLLVM_TARGETS_TO_BUILD=${LLVM_TARGETS_TO_BUILD}
         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
+        -DLLVM_EXTERNAL_CLANG_SOURCE_DIR=${LLVM_EXTERNAL_CLANG_SOURCE_DIR}
       WORKING_DIRECTORY ${LLVM_${target_name}_BUILD} )
   endif(NOT IS_DIRECTORY ${LLVM_${target_name}_BUILD})
 
 endfunction()
 
 function(llvm_create_cross_target target_name sysroot)
   llvm_create_cross_target_internal(${target_name} ${sysroot} ${CMAKE_BUILD_TYPE})
 endfunction()
 
 llvm_create_cross_target_internal(NATIVE "" Release)