[OpenMP] Fixed an issue that wrong LLVM headers might be included when building libomptarget

[tianshilei1992]

Summary

Wrong LLVM headers might be included if we don't set include_directories
to a right place. This will cause a compilation error if LLVM is installed in
system directories.

[jdoerfert]

LGTM.

openmp/libomptarget/src/CMakeLists.txt

set(LIBOMPTARGET_SRC_FILES
   ${CMAKE_CURRENT_SOURCE_DIR}/api.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/device.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/interface.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/MemoryManager.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/rtl.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/omptarget.cpp
 )
 
+include_directories(${LIBOMPTARGET_LLVM_MAIN_INCLUDE_DIR})
+
 # Build libomptarget library with libdl dependency. Add LLVMSupport
 # dependency if building in-tree with profiling enabled.
 if(OPENMP_STANDALONE_BUILD OR (NOT OPENMP_ENABLE_LIBOMPTARGET_PROFILING))
   add_library(omptarget SHARED ${LIBOMPTARGET_SRC_FILES})
   target_link_libraries(omptarget
     ${CMAKE_DL_LIBS}
     "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/exports")
 else()