message: [unittests] Use object library if cmake supports it
diff: -add_library(lldb-mi-utils OBJECT
  +# Create object library to avoid unnecessary linking. If CMake version
  +# doesn't support it, just create an ordinary library.
  +if(NOT (${CMAKE_VERSION} VERSION_LESS "3.5.0"))
  + set (LIBRARY_TYPE "OBJECT")
  +endif()
  +
  +add_library(lldb-mi-utils ${LIBRARY_TYPE}
  ${LLDB_SOURCE_DIR}/tools/lldb-mi/MIUtilString.cpp
  )
  @@ -9,5 +15,16 @@ add_lldb_unittest(LLDBMiUtilTests
  Support
  )
  +if((${CMAKE_VERSION} VERSION_LESS "3.5.0") OR
  + NOT (${CMAKE_VERSION} VERSION_LESS "3.12.0"))
  + # Link to either usual (cmake version < 3.5) or
  + # object (cmake version >= 3.12) library with lldm-mi sources.
  + target_link_libraries(LLDBMiUtilTests PRIVATE lldb-mi-utils)
  +else()
  + # Object libraries still cannot be on the right-hand side of
  + # `target_link_libraries` but `$<TARGET_OBJECTS:objlib>` expression is
  + # already supported in `target_sources`.
  target_sources(LLDBMiUtilTests PRIVATE $<TARGET_OBJECTS:lldb-mi-utils>)
  +endif()
  +
  set_target_properties(lldb-mi-utils PROPERTIES FOLDER "lldb libraries")
  