Revert the changes from commit a2a90f41e3 (Tests: require C++14 for the
Tutorial, 2019-03-21, v3.15.0-rc1~41^2~2) for the content in its new
home. In commit d50b31be35 (Clang: For MSVC ABI do not use modes older
than C++14, 2019-07-23) we fixed the C++ standard selection for GNU-like
Clang with the MSVC ABI so the test code itself no longer needs to do
it. In particular, changing the tests in this way broke the tutorial's
narrative.
Migrate tutorial text from individual `directions.txt` files to the main
tutorial document. Add some comments to source code to provide anchors
for inclusion.
Create a `Help/guide/` directory to hold guide-level documents.
Build them in most documentation formats, but not as man pages.
Initialize the guide directory with a placeholder for the tutorial.
