message: [unittests] Simplify CMakeLists with object library
  The solution suggested by Chris Bieneman works for all versions of CMake.
diff: -# 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}
  +add_library(lldb-mi-utils OBJECT
  ${LLDB_SOURCE_DIR}/tools/lldb-mi/MIUtilString.cpp
  )
  add_lldb_unittest(LLDBMiUtilTests
  + $<TARGET_OBJECTS:lldb-mi-utils>
  StringTest.cpp
  LINK_COMPONENTS
  Support
  )
  -if((${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")
  