Fully emulating type_caster_base-related behavior: trying shared_from_this also for unowned pointees.

enable_shared_from_this_from_raw_ptr_take_ownership_guard: better static_assert that also triggers for derived classes.

Restoring init_holder overload for std::enable_shared_from_this and original tests/test_smart_ptr.py.

Breaking up test_shared_ptr_from_this_and_references into smaller subtests. Skipping the only test that generates an ASAN heap-use-after-free.

Experiments: 1. disabling enable_shared_from_this, 2. using smart_holder in test_class_sh_shared_from_this.cpp

* Allow casting from None to a custom object, closes #2778

* ci.yml patch from the smart_holder branch for full CI coverage.

This also disables catch for some other builds that happen to be in the same
matrix. But there are still plenty of builds running with DOWNLOAD_CATCH=ON.

Workaround for a windows-only download issue that started yesterday (2020-06-23):

-- Downloading catch v2.13.2...
CMake Error at tools/FindCatch.cmake:40 (message):
  Could not download
  https://github.com/philsquared/Catch/releases/download/v2.13.2/catch.hpp
Call Stack (most recent call first):
  tools/FindCatch.cmake:59 (_download_catch)
  tests/pure_cpp/CMakeLists.txt:1 (find_package)

* Updating and slightly enhancing instructions for running clang-tidy.

* Making documented commands identical to workflow commands. Adding comment in workflow file pointing to documentation.

More clang-tidy fixes. These escaped before because `-DDOWNLOAD_EIGEN=ON -DDOWNLOAD_CATCH=ON -DCMAKE_CXX_STANDARD=17`, as used in the GitHub Actions, were missing in the interactive run.

* Initial fixes

* Whoops

* Finish clang-tidy manual fixes

* Add two missing fixes

* Revert

* Update clang-tidy

* Try to fix unreachable code error

* Move nolint comment

* Apply missing fix

* Don't override clang-tidy config

* Does this fix clang-tidy?

* Make all clang-tidy errors visible

* Add comments about NOLINTs and remove a few

* Fix typo

* clang-tidy fixes (related to recent clang-tidy changes on master).

* More clang-tidy fixes.

* Enable and apply clang-tidy readability and misc fixes.

* Revert deprecated tester

* add space to tests/test_constants_and_functions.cpp