[NFC][CMake] Add FOLDER to utility targets

As Visual Studio's CMake support is getting better and better the line
between IDE generator and non-IDE generators is blurring. Visual Studio
2019 and later have a very useful UI that can handle all of the various
targets we create, but if they are unsorted it is wildly unwieldy.

This change sorts the lit testsuite targets and per-component install
targets into folders, which are not generated for IDE generators but
are generated by default under Visual Studio's CMake + Ninja
integration.